Drainage Design Manual - New Jersey€¦ · Drainage Design Manual August 2006 . Drainage Design...

State of New Jersey Department of Transportation

Drainage Design Manual

August 2006

Drainage Design

1.0 General Information ............................................................... 3

2.0 Drainage Policy ...................................................................... 7

3.0 Hydrology ........................................................................... 14

4.0 Channel Design ................................................................... 32

5.0 Drainage of Highway and Pavements ...................................... 36

6.0 Storm Drains ....................................................................... 46

7.0 Median Drainage .................................................................. 62

8.0 Culvert Design ..................................................................... 65

9.0 Conduit Outlet Protection ...................................................... 70

10.0 Reset Castings - Manholes and Inlets ..................................... 72

11.0 Stormwater Management ...................................................... 75

12.0 Water Quality ...................................................................... 82

13.0 Sample Hydrologic and Hydraulic Calculations ......................... 86

References ........................................................................ 100

Drainage Design

1.0 General Information

1.1 Introduction

Investigation of the impacts of surface water on the highway roadway,

channels, and surrounding land is an integral part of every highway design. The

end product of this investigation is a design, included in the plans, that provides

an economical means of accommodating surface water to minimize adverse

impacts in accordance with the design procedures.

Traffic safety is intimately related to surface drainage. Rapid removal of

stormwater from the pavement minimizes the conditions which can result in the

hazardous phenomenon of hydroplaning. Adequate cross-slope and longitudinal

grade enhance such rapid removal. Where curb and gutter are necessary, the

provision of sufficient inlets in conjunction with satisfactory cross-slope and

longitudinal slope are necessary to efficiently remove the water and limit the

spread of water on the pavement. Inlets at strategic points on ramp

intersections and approaches to superelevated curves will reduce the likelihood

of gutter flows spilling across roadways. Satisfactory cross-drainage facilities

will limit the buildup of ponding against the upstream side of roadway

embankments and avoid overtopping of the roadway.

Stormwater management is an increasingly important consideration in the

design of roadway drainage systems. Existing downstream conveyance

constraints, particularly in cases where the roadway drainage system connects

to existing pipe systems, may warrant installation of detention/recharge basins

to limit the peak discharge to the capacity of the downstream system. Specific

stormwater management requirements to control the rate and volume of runoff

may be dictated by various regulatory agencies.

Water quality is also an increasingly important consideration in the design of

roadway drainage systems, particularly as control of non-point source pollution

is implemented. Specific water quality requirements may be dictated by various

regulatory agencies.

Detailed requirements regarding water quality control are included in Section

12.0 of this Manual and the separate document prepared by the New Jersey

Department of Environmental Protection (NJDEP) entitled Stormwater Best

Management Practices Manual.

The optimum roadway drainage design should achieve a balance among public

safety, the capital costs, operation and maintenance costs, public convenience,

environmental enhancement and other design objectives.

The purpose of this manual is to provide the technical information and

procedures required for the design of culverts, storm drains, channels, and

stormwater management facilities. This section contains design criteria and

information that will be required for the design of highway drainage structures.

The complexity of the subject requires referring to additional design manuals

and reports for more detailed information on several subjects.

1.2 Definitions and Abbreviations

Following is a list of important terms which will be used throughout this volume.

Drainage Design

AWS - Allowable water surface elevations - The water surface elevation above

which damage will occur.

AHW - Allowable headwater elevation - The allowable water surface elevation

upstream from a culvert.

Backwater - The increased depth of water upstream from a dam, culvert, or

other drainage structure due to the existence of such obstruction.

Best Management Practice (BMP) – A structural feature or non-structural

development strategy designed to minimize or mitigate for impacts associated

with stormwater runoff, including flooding, water pollution, erosion and

sedimentation, and reduction in groundwater recharge.

Bioretention – A water quality treatment system consisting of a soil bed

planted with native vegetation located above an underdrained sand layer. It

can be configured as either a bioretention basin or a bioretention swale.

Stormwater runoff entering the bioretention system is filtered first through the

vegetation and then the sand/soil mixture before being conveyed downstream

by the underdrain system.

Category One Waters – Those waters designated in the tables in N.J.A.C.

7:9B-4.15(c) through (h) for the purposes of implementing the Antidegradation

Polices in N.J.A.C. 7:9B-4. These waters received special protection under the

Surface Water Quality Standards because of their clarity, color, scenic setting

or other characteristics of aesthetic value, exceptional ecological significance,

exceptional recreational significance, exceptional water supply significance or

exceptional fisheries resource(s). More information on Category One Waters can

be found on the New Jersey Department of Environmental Protection’s (NJDEP)

web sites http://www.state.nj.us/dep and

http://www.state.nj.us/dep/antisprawl/c1.html.

Channel - A perceptible natural or artificial waterway which periodically or

continuously contains moving water. It has a definite bed and banks which

confine the water. A roadside ditch, therefore, would be considered a channel.

Culvert – A hydraulic structure that is typically used to convey surface waters

through embankments. A culvert is typically designed to take advantage of

submergence at the inlet to increase hydraulic capacity. It is a structure, as

distinguished from a bridge, which is usually covered with embankment and is

composed of structural material around the entire perimeter, although some

are supported on spread footings with the stream bed serving as the bottom of

the culvert. Culverts are further differentiated from bridges as having spans

typically less than 25 feet.

Dam - Any artificial dike, levy or other barrier together with appurtenant

works, which impounds water on a permanent or temporary basis, that raises

the water level 5 feet or more above its usual mean low water height when

measured from the downstream toe-of-dam to the emergency spillway crest or,

in the absence of an emergency spillway, to the top of dam.

Design Flow - The flow rate at a selected recurrence interval.

Fluvial Flood - A flood which is caused entirely by runoff from rainfall in the

upstream drainage area and is not influenced by the tide or tidal surge.

Floodplain - The area described by the perimeter of the Design Flood. That

portion of a river valley which has been covered with water when the river

overflowed its banks at flood stage. An area designated by a governmental

agency as a floodplain.

http://www.state.nj.us/dep

http://www.state.nj.us/dep/antisprawl/c1.html

Drainage Design

Pipe - A conduit that conveys stormwater which is intercepted by the inlets, to

an outfall where the stormwater is discharged to the receiving waters. The

drainage system consists of differing lengths and sizes of pipe connected by

drainage structures.

Recurrence Interval - The average interval between floods of a given

magnitude.

Regulatory Flood – For delineated streams (i.e., those for which a State

Adopted Flood Study exists), it is the Flood Hazard Area Design Flood, which is

the 100-year peak discharge increased by 25 percent. State Adopted Flood

Studies can be obtained from the NJDEP Bureau of Floodplain Management. For

non-delineated streams, it is the 100-year peak discharge, based on fully

developed conditions within the watershed.

Scour – Erosion of stream bed or bank material due to flowing water; often

considered as being localized.

Stream Encroachment - Any manmade alteration, construction, development,

or other activity within a floodplain.

Time of Concentration (Tc) – Time required for water to flow from the most

hydraulically distant (but hydraulically significant) point of a watershed, to the

outlet.

Total Suspended Solids (TSS) - Solids in water that can be trapped by a

filter, which include a wide variety of material, such as silt, decaying plant and

animal matter, industrial wastes, and sewage.

1.3 Design Procedure Overview

This chapter outlines the general process of design for roadway drainage

systems. Detailed information regarding drainage design is included in the

remainder of this Manual.

A. Preliminary investigation will be performed using available record data,

including reports, studies, plans, topographic maps, etc., supplemented with

field reconnaissance. Information should be obtained for the project area

and for adjacent stormwater management projects that may affect the

highway drainage.

B. Site Analysis: At each site where a drainage structure(s) will be

constructed, the following items should be evaluated as appropriate from

information given by the preliminary investigation:

1. Drainage Area.

2. Land Use.

3. Allowable Headwater.

4. Effects of Adjacent Structures (upstream and downstream).

5. Existing Streams and Discharge Points.

6. Stream Slope and Alignment.

7. Stream Capacity.

8. Soil Erodibility.

9. Environmental permit concerns and constraints.

Coordination with representatives of the various environmental disciplines is

encouraged.

Drainage Design

C. Recurrence Interval: Select a recurrence interval in accordance with the

design policy set forth in Section 2.0.

D. Hydrologic Analysis: Compute the design flow utilizing the appropriate

hydrologic method outlined in Section 3.0.

E. Hydraulic Analysis: Select a drainage system to accommodate the design

flow utilizing the procedures outlined in the following parts:

1. Channel Design – Section 4.0

2. Drainage of Highway Pavements – Section 5.0

3. Storm Drains - Section 6.0

4. Median Drainage – Section 7.0

5. Culverts - Section 8.0

F. Environmental Considerations: Environmental impact of the proposed

drainage system and appropriate methods to avoid or mitigate adverse

impacts should be evaluated. Items to be considered include:

1. Stormwater Management

2. Water Quality

3. Soil Erosion and Sediment Control

4. Special Stormwater Collection Procedures

5. Special Stormwater Disposal Procedures

These elements should be considered during the design process and

incorporated into the design as it progresses.

G. Drainage Review: The design engineer should inspect the drainage system

sites to check topography and the validity of the design. Items to check

include:

1. Drainage Area

a. Size

b. Land Use

c. Improvements

2. Effects of Allowable Computed Headwater

3. Performance of Existing or Adjacent Structures

a. Erosion

b. Evidence of High Water

4. Channel Condition

a. Erosion

b. Vegetation

c. Alignment of Proposed Facilities with Channel

5. Impacts on Environmentally Sensitive Areas

Drainage Design

2.0 Drainage Policy

2.1 Introduction

This part contains procedures and criteria that are essential for roadway

drainage design.

2.2 Stormwater Management and Non-Point Source Pollution Control

Stormwater is a component of the total water resources of an area and should

not be casually discarded but rather, where feasible, should be used to

replenish that resource. In many instances, stormwater problems signal either

misuse of a resource or unwise land activity.

Poor management of stormwater increases total flow, flow rate, flow velocity

and depth of water in downstream channels. In addition to stormwater peak

discharge and volume impacts, roadway construction or modification usually

increases non-point source pollution primarily due to the increased impervious

area. Properly designed stormwater management facilities, particularly

detention/recharge basins, can also be used to mitigate non-point source

pollution impacts by providing extended containment duration, thereby allowing

settlement of suspended solids. Sections 2.6, 11.0 and 12.0 of this Manual and

the Stormwater Best Management Practices Manual prepared by the New Jersey

Department of Environmental Protection (NJDEP) provide the guidance in the

planning and design of these facilities.

An assessment of the impacts the project will have on existing peak flows and

watercourses shall be made by the design engineer during the initial phase. The

assessment shall identify the need for stormwater management and non-point

source pollution control (SWM & NPSPC) facilities and potential locations for

these facilities. Mitigating measures can include, but are not limited to,

detention/recharge basins, grassed swales, channel stabilization measures, and

easements.

Stormwater management, whether structural or non-structural, on or off site,

must fit into the natural environment, and be functional, safe, and aesthetically

acceptable. Several alternatives to manage stormwater and provide water

quality may be possible for any location. Careful design and planning by the

engineer, hydrologist, biologist, environmentalist, and landscape architect can

produce optimum results.

Design of SWM & NPSPC measures must consider both the natural and man-

made existing surroundings. The design engineer should be guided by this and

include measures in design plans that are compatible with the site specific

surroundings. Revegetation with native, non-invasive grasses, shrubs and

possibly trees may be required to achieve compatibility with the surrounding

environment. Design of major SWM & NPSPC facilities may require coordination

with the New Jersey Department of Transportation (NJDOT)-Landscape and

Urban Design Unit, Bureau of Environmental Services, and other state and

various regulatory agencies.

SWM & NPSPC facilities shall be designed in accordance with Sections 11.0 and

12.0 and the Stormwater Best Management Practices Manual prepared by the

NJDEP or other criteria where applicable, as directed by the Department.

Drainage Design

Disposal of roadway runoff to available waterways that either cross the

roadway or are adjacent to it spaced at large distances, requires installation of

long conveyance systems. Vertical design constraints may make it impossible to

drain a pipe or swale system to existing waterways. Discharging the runoff to

the groundwater with a series of leaching or seepage basins (sometimes called

a Dry Well) may be an appropriate alternative if groundwater levels and non-

contaminated, permeable soil conditions allow a properly designed system to

function as designed. The decision to select a seepage facility design must

consider geotechnical, maintenance, and possibly right-of-way (ROW) impacts

and will only be allowed if no alternative exists.

The seepage facilities must be designed to store the entire runoff volume for a

design storm compatible with the storm frequency used for design of the

roadway drainage facilities or as directed by the Department. As a minimum,

the seepage facilities shall be designed to store the increase in runoff volume

from new impervious surfaces as long as adequate overflow conveyance paths

are available to safely carry the larger flows to a stable discharge point.

Installation of seepage facilities can also satisfy runoff volume control and

water quality concerns which may be required by an environmental permit.

Additional design guidelines are included in the NJDEP Stormwater Best

Management Practices Manual.

2.3 Allowable Water Surface Elevation

Determine the allowable water surface elevation (AWS) at every site where a

drainage facility will be constructed. The proposed drainage structure should

cause a ponding level, hydraulic grade line elevation, or backwater elevation no

greater than the AWS when the design flow is imposed on the facility. The AWS

must comply with NJDEP requirements for locations that require a Stream

Encroachment Permit. The AWS upstream of a proposed drainage facility at

locations that do not require a Stream Encroachment Permit should not cause

additional flooding outside the NJDOT property or acquired easements. An AWS

that exceeds a reasonable limit may require concurrence of the affected

property owner.

A floodplain study prepared by the New Jersey Department of Environmental

Protection, the Federal Emergency Management Agency, the U.S. Army Corps

of Engineers, or other recognized agencies will be available at some sites. The

elevations provided in the approved study will be used in the hydraulic model.

The Table 2-1 presents additional guidelines for determining the AWS at

locations where a Stream Encroachment Permit is not required.

Drainage Design

Table 2-1

Allowable Water Surface (AWS)

Land Use or Facility AWS

Residence Floor elevation (slab floor), basement window,

basement drain (if seepage potential is

present)

Commercial Building (barn, store,

warehouse, office building, etc.)

Same as for residence

Bridge Low steel

Culvert Top of culvert - New structure

Outside edge of road - Existing structure

Levee Min 1 foot below top of Levee

Dam See NJDEP Dam Safety Standards

Channel Min 1 foot below top of low bank

Road Min 1 foot below top of grate or manhole rim

for storm sewers

The peak 100-year water surface elevation for any new detention/retention

facility must be contained within NJDOT property or acquired easements. No

additional flooding shall result outside the NJDOT property or acquired

easements.

2.4 Recurrence Interval

Select a flood recurrence interval consistent with Table 2-2:

Table 2-2

Recurrence Interval

Recurrence

Interval Facility Description

100-Year Any drainage facility that requires a NJDEP permit for a non-

delineated stream. For delineated watercourses contact the

NJDEP Bureau of Floodplain Management.

50-Year Any drainage structure that passes water under a freeway or

interstate highway embankment, with a headwall or open end at

each side of the roadway.

25-Year Any drainage structure that passes water under a land service

highway embankment, with a headwall or open end at each side

of the roadway. Also, pipes along the mainline of a freeway or

interstate highway that convey runoff from a roadway low point

to the disposal point, a waterway, or a stormwater maintenance

facility.

15-Year Longitudinal systems and cross drain pipes of a freeway or

interstate highway. Also pipes along mainline of a land service

highway that convey runoff from a roadway low point to the

disposal point, a waterway, or a stormwater maintenance

facility.

10-Year Longitudinal systems and cross drain pipes of a land service

highway.

Drainage Design

2.5 Increasing Fill Height Over Existing Structures

Investigate the structural adequacy of existing structures that will have

additional loading as the result of a surcharge placement or construction loads.

2.6 Regulatory Compliance

Proposed construction must comply with the requirements of various regulatory

agencies. Depending on the project location, these agencies could include, but

are not limited to, the US Army Corps of Engineers, U. S. Coast Guard, the New

Jersey Department of Environmental Protection, the Pinelands Commission, the

Highlands Council and the Delaware and Raritan Canal Commission.

The NJDEP has adopted amendments to the New Jersey Pollution Discharge

Elimination System (NJPDES) program to include a Construction Activity

Stormwater General Permit (NJ 0088323). This program is administered by the

NJ Department of Agriculture through the Soil Conservation Districts (SCD).

Certification by the local SCD is not required for NJDOT projects. However,

certification by the local SCD is required for non-NJDOT projects (i.e., a County

is the applicant). A Request for Authorization (RFA) for a NJPDES Construction

Stormwater Permit is needed only for non-NJDOT projects that disturb more

than one (1) acre and must be submitted to the local SCD.

The NJDEP has adopted the New Jersey Stormwater Management Rule, N.J.A.C.

7.8. The Stormwater Management Rule governs all projects that provide for

ultimately disturbing one (1) or more acres of land or increasing impervious

surface by 0.25 acre or more. The following design and performance standards

need to be addressed for any project governed by the Stormwater Management

Rule:

Nonstructural Stormwater Management Strategies, N.J.A.C. 7:8-5.3

To the maximum extent possible, nonstructural stormwater BMPs shall

be used to meet the requirements of the Stormwater Management Rule.

If the design engineer determines that it is not feasible for engineering,

environmental or safety reason to utilize nonstructural stormwater

BMPs, structural BMPs may be utilized.

Groundwater Recharge, N.J.A.C. 7:8-5.4(a)2

For the project, the design engineer shall demonstrate either that the

stormwater BMPs maintain 100% of the average annual preconstruction

groundwater recharge volume for the site; or that the increase in

stormwater runoff volume from pre-construction to post-construction for

the 2-year storm is infiltrated. NJDEP has provided an Excel Spreadsheet

to determine the project sites annual groundwater recharge amounts in

both pre- and post-development site conditions. A full explanation of the

spreadsheet and its use can be found in Chapter 6 of the New Jersey

Stormwater Best Management Practices Manual. A copy of the

spreadsheet can be downloaded from http://www.njstormwater.org.

Stormwater Quantity, N.J.A.C. 7:8-5.4(a)3

Stormwater BMPs shall be designed to do one of the following:

1. The post-construction hydrograph for the 2-year, 10-year, and

100-year storm events do not exceed, at any point in time, the

pre-construction runoff hydrographs for the same storm events.

2. There shall be no increase, as compared to the pre-construction

condition, in peak runoff rates of stormwater leaving the project

Drainage Design

site for the 2-year, 10-year, and 100-year storm events and that

the increased volume or change in timing of stormwater runoff

will not increase flood damage at or downstream of the site. This

analysis shall include the analysis of impacts of exiting land uses

and projected land uses assuming full development under

existing zoning and land use ordinances in the drainage area.

3. The post-construction peak runoff rates for the 2-year, 10-year,

and 100-year storm events are 50%, 75%, and 80%,

respectively, of the pre-construction rates. The percentages apply

only to the post-construction stormwater runoff that is attributed

to the portion of the site on which the proposed development or

project is to be constructed.

4. Along tidal or tidally influenced waterbodies and/or in tidal

floodplains, stormwater runoff quantity analysis shall only be

applied if the increased volume of stormwater runoff could

increase flood damages below the point of discharge. Tidal

flooding is the result of higher than normal tides which in turn

inundate low lying coastal areas. Tidal areas are not only

activities in tidal waters, but also the area adjacent to the water,

including fluvial rivers and streams, extending from the mean

high water line to the first paved public road, railroad or

surveyable property line. At a minimum, the zone extends at

least 100 feet but no more than 500 feet inland from the tidal

water body.

Stormwater Quality, N.J.A.C. 7:8-5.5

Stormwater BMPs shall be designed to reduce the post-construction load

of TSS in stormwater runoff generated from the water quality storm by

80% of the anticipated load from the developed site. Section 12 and the

Stormwater Best Management Practices Manual provide guidance in the

planning and design of these facilities.

Stormwater Maintenance Plan, N.J.A.C. 7:8-5.8

The design engineer shall prepare a stormwater management facility

maintenance plan in accordance with the New Jersey Stormwater Rule.

At a minimum the maintenance plan shall include specific preventative

maintenance tasks and schedules. Maintenance guidelines for

stormwater management measures are available in the New Jersey

Stormwater Best Management Practices Manual.

For projects located within the Pinelands or Highlands areas of the State, the

design engineer should consult with the NJDEP to determine what additional

stormwater management requirements may apply to the project. Additional

information about the Pinelands can be found at

http://www.state.nj.us/pinelands/, and information about the Highlands can be

found at http://www.nj.gov/dep/highlands/.

On NJDOT projects, a RFA does not have to be sent to the SCD, but instead the

environmental team sends a notification directly to the NJDEP. A RFA would

have to be sent to the appropriate Soil Conservation District only for non-

NJDOT projects (i.e. a County is the applicant).

The NJDOT Bureau of Environmental Services will provide guidance regarding

project specific permit requirements. Guidance regarding NJDEP Stream

Encroachment Permits is provided in Section 2.7.

Drainage Design

2.7 Stream Encroachment

Stream Encroachment Permits for which the NJDOT is the applicant shall be

processed in accordance with Section 13 of the NJDOT Procedures Manual and

the following guidelines.

Applicability and specific requirements for all Stream Encroachment Permits

may be found in the most recent Flood Hazard Area Control Act Rules as

adopted by the New Jersey Department of Environmental Protection (NJDEP).

Specific requirements for bridges and culverts are contained in N.J.A.C. 7.13 -

2.16.

In cases where the regulatory flood causes the water surface to overflow the

roadway, the design engineer shall, by raising the profile of the roadway, by

increasing the size of the opening or a combination of both, limit the water

surface to an elevation equal to the elevation of the outside edge of shoulder.

The design engineer is cautioned, however, to critically assess the potential

hydrologic and hydraulic effects upstream and downstream of the project,

which may result from impeding flow by raising the roadway profile, or from

decreasing upstream storage and allowing additional flow downstream by

increasing existing culvert openings. The design engineer shall determine what

effect the resulting reduction of storage will have on peak flows and the

downstream properties in accordance with the Flood Hazard Area Control Act

Rules. Stormwater management facilities may be required to satisfy these

requirements.

N.J.A.C. 7:13 - 2.3(b)1. indicates that the discharge for non-delineated

watercourses is to be based on ultimate development in accordance with the

current zoning plan. Hydraulic evaluation of existing roadway stream crossings

may reveal that the water surface elevation for this discharge overtops the

roadway. Compliance with both the bridge and culvert requirements presented

in N.J.A.C. 7:13 - 2.16 and the NJDOT requirement to avoid roadway

overtopping may require coordination between the agencies involved to achieve

a reasonable design approach. In addition to the regulations listed above, the

bridge and culvert design will be in compliance with the NJDEP’s Technical

Manual for Land Use Regulation Program, Bureaus of Inland and Coastal

Regulations, Stream Encroachment Permits, which includes the following:

Structures will pass the regulatory flood without increasing the upstream

elevation of the flood profile by more than 0.2 feet if the structure is

new or the upstream and downstream flood profile by more than 0.0

feet if the structure is a replacement for an existing structure.

For new structures that result in lowering the downstream water surface

elevation by 2 or 3 feet, the engineer must perform a routing analysis to

verify that there are no adverse impacts further downstream.

Activities located along tidal waterbodies listed in the Flood Hazard Area Control

Act Rules are not governed by NJDEP, Land Use Regulation Program, Stream

Encroachment Section; however, a permit may be required from another unit of

the NJDEP.

When a permit is required, the NJDOT Drainage Engineer shall be notified in

writing. This notice shall include a USGS Location Map with the following

information:

Drainage Design

a. A title block identifying the project by name, the applicant, and the name of

the quadrangle.

b. The limits of the project and point of encroachment shown in contrasting

colors on the map.

c. The upstream drainage area contributing runoff shall be outlined for all

streams and/or swales within or along the project.

If the NJDOT Project Engineer, after consultation with NJDEP, determines

that a pre-application meeting is desirable, the following engineering data

may also be required for discussion at a NJDEP pre-application meeting.

d. A 1" = 30' scale plan with the encroachment location noted thereon.

e. In the case of a new or replacement structure or other type encroachment,

the regulatory floodwater surface elevation as required for the review and

analysis of the project impacts and permit requirements.

The design engineer is also required to determine whether a particular

watercourse involved in the project is classified by the State as a Category One

waterbody, and if so, shall design the project in accordance with the provisions

at N.J.A.C. 7:9B-4. Projects involving a Category One waterbody shall be

designed such that a 300-foot special water resource protection area is

provided on each side of the waterbody. Encroachment within this 300-foot

buffer is prohibited except in instances where preexisting disturbance exists.

Where preexisting disturbance exists, encroachment is allowed, provided that

the 95% TSS removal standard is met and the loss of function is addressed.

More information on Category One Waters can be found

in the NJDEP’s web sites http://www.state.nj.us/dep or

http://www.state.nj.us/dep/antisprawl/c1.html.

2.8 Soil Erosion and Sediment Control

The design for projects that disturb 5,000 or more square feet do not require

plan certification from the local Soil Conservation District, but shall be prepared

in accordance with the current version of the NJDOT Soil Erosion and Sediment

Control Standards, including the required report. The Soil Erosion and Sediment

Control Report shall include calculations and plans that address both temporary

and permanent items for the engineering and vegetative standards.

Calculations shall be shown for items that require specific sizing (e.g., rip rap,

settling basins, etc.). Certification by the local Soil Conservation District is not

required for NJDOT projects. Certification by the local Soil Conservation District

is required for non-NJDOT projects (i.e., a County is the applicant).

http://www.state.nj.us/dep

http://www.state.nj.us/dep/antisprawl/c1.html

Drainage Design

3.0 Hydrology

3.1 Introduction

Hydrology is generally defined as a science dealing with the interrelationship

between water on and under the earth and in the atmosphere. For the purpose

of this section, hydrology will deal with estimating flood magnitudes as the

result of precipitation. In the design of highway drainage structures, floods are

usually considered in terms of peak runoff or discharge in cubic feet per second

(cfs) and hydrographs as discharge per time. For drainage facilities which are

designed to control volume of runoff, like detention facilities, or where flood

routing through culverts is used, then the entire discharge hydrograph will be of

interest. The analysis of the peak rate of runoff, volume of runoff, and time

distribution of flow is fundamental to the design of drainage facilities. Errors in

the estimates will result in a structure that is either undersized and causes

more drainage problems or oversized and costs more than necessary.

In the hydrologic analysis for a drainage facility, it must be recognized that

many variable factors affect floods. Some of the factors which need to be

recognized and considered on an individual site by site basis include:

rainfall amount and storm distribution,

drainage area size, shape and orientation, ground cover, type of soil,

slopes of terrain and stream(s),

antecedent moisture condition,

storage potential (overbank, ponds, wetlands, reservoirs, channel, etc.),

watershed development potential, and

type of precipitation (rain, snow, hail, or combinations thereof), elevation.

The type and source of information available for hydrologic analysis will vary

from site to site. It is the responsibility of the design engineer to determine the

information required for a particular analysis. This subsection contains

hydrologic methods by which peak flows and hydrographs may be determined

for the hydraulic evaluation of drainage systems of culverts, channels and

median drains.

Drainage Design

3.2 Selection of Hydrologic Methods

The following guidelines should be used to select the hydrology method for

computing the design peak flow:

Table 3-1

Hydrologic Method

Size of Drainage Area Hydrologic Method‡

Less than 20 Acres Rational Formula or Modified

Rational Method

Less than 5 Square Miles NRCS* TR-55 Methodology

Greater than 1 Acre NRCS* TR-20, HEC-1 Method,

HEC-HMS or others†

‡ For all projects in certain areas south of the South

Central flat inland and New Jersey Coastal Plain, the

DELMARVA Unit Hydrograph shall be incorporated into

the design procedure. Contact the local Soil Conservation

District to determine if the DELMARVA unit hydrograph is

to be used for the project. * US Natural Resources Conservation Service (NRCS),

formerly the US Soil Conservation Service (SCS). These hydrologic models are not limited by the size of

the drainage area. They are instead limited by uniform

curve number, travel time, etc. Most of these limitations

can be overcome by subdividing the drainage areas into

smaller areas. See the appropriate users manual for a

complete list of limitations for each hydrologic model. † Many hydrologic models exist beyond those that are

listed here. If a model is not included, then the design

engineer should ensure that the model is appropriate

and that approvals are obtained from the Department.

The peak flow from a drainage basin is a function of the basin’s physiographic

properties such as size, shape, slope, soil type, land use, as well as

climatological factors such as mean annual rainfall and selected rainfall

intensities. The methods presented in the guideline should give acceptable

predictions for the indicated ranges of drainage area sizes and basin

characteristics.

Other hydrologic methods may be used only with the approval of the

Department.

NOTE: If a watercourse has had a NJDEP adopted study prepared for the

particular reach where the project is located, that study should be used for the

runoff and water surface profiles. NJDEP does not accept FEMA studies, since

the FEMA hydrologic models do not consider that the entire drainage area is to

be fully developed. The design engineer should ensure that the hydrologic

model used takes into account the NJDEP requirement that the entire upstream

drainage area is to be considered fully developed.

Computation of peak discharge must consider the condition that yields the

largest rate. Proper hydrograph combination is essential. It may be necessary

Drainage Design

to evaluate several different hydrograph combinations to determine the peak

discharge for basins containing hydrographs with significantly different times

for the peak discharge. For example, the peak discharge for a basin with a large

undeveloped area contributing toward the roadway may result from either the

runoff at the time when the total area reaches the roadway or the runoff from

the roadway area at its peak time plus the runoff from the portion of the

overland area contributing at the same time.

3.3 Rational Formula

The rational formula is an empirical formula relating runoff to rainfall intensity.

It is expressed in the following form:

Q = CIA

Where:

Q = peak flow in cubic feet per second ft3/s

C = runoff coefficient (weighted)

I = rainfall intensity in inches (in) per hour

A = drainage area in acres

A. Basic Assumptions

1. The peak rate of runoff (Q) at any point is a direct function of the

average rainfall intensity (I) for the Time of Concentration (Tc) to that

point.

2. The recurrence interval of the peak discharge is the same as the

recurrence interval of the average rainfall intensity.

3. The Time of Concentration is the time required for the runoff to become

established and flow from the most distant point of the drainage area to

the point of discharge.

A reason to limit use of the rational method to small watersheds pertains to

the assumption that rainfall is constant throughout the entire watershed.

Severe storms, say of a 100-year return period, generally cover a very

small area. Applying the high intensity corresponding to a 100-year storm

to the entire watershed could produce greatly exaggerated flows, as only a

fraction of the area may be experiencing such an intensity at any given

time.

The variability of the runoff coefficient also favors the application of the

rational method to small, developed watersheds. Although the coefficient is

assumed to remain constant, it actually changes during a storm event. The

greatest fluctuations take place on unpaved surfaces as in rural settings. In

addition, runoff coefficient values are much more difficult to determine and

may not be as accurate for surfaces that are not smooth, uniform and

impervious.

To summarize, the rational method provides the most reliable results when

applied to small, developed watersheds and particularly to roadway

drainage design. The validity of each assumption should be verified for the

site before proceeding.

Drainage Design

B. Procedure

1. Obtain the following information for each site:

a. Drainage area

b. Land use (% of impermeable area such as pavement, sidewalks or

roofs)

c. Soil types (highly permeable or impermeable soils)

d. Distance from the farthest point of the drainage area to the point of

discharge

e. Difference in elevation from the farthest point of the drainage area to

the point of discharge

2. Determine the Time of Concentration (Tc). See Section 3.5.

(Minimum Tc is 10 minutes).

3. Determine the rainfall intensity rate (I) for the selected recurrence

intervals.

4. Select the appropriate C value.

5. Compute the design flow (Q = CIA).

The runoff coefficient (C) accounts for the effects of infiltration, detention

storage, evapo-transpiration, surface retention, flow routing and

interception. The product of C and the average rainfall intensity (I) is the

rainfall excess of runoff per acre.

The runoff coefficient should be weighted to reflect the different conditions

that exist within a watershed.

Example:

Cw =

A1C1+ A2C2 . . . ANCN

A1 + A2 . . . AN

C. Value for C: Select the appropriate value for C from Table 3-2:

Drainage Design

Table 3-2

Recommended Coefficient of Runoff Values

for Various Selected Land Uses

Land Use Description Hydrologic Soils Group

A B C D

Cultivated Land without conservation treatment

with conservation treatment

0.49

0.27

0.67

0.43

0.81

0.67

0.88

0.67

Pasture or Range Land

Meadow

poor condition

good condition

good condition

0.38

---

---

0.63

0.25

---

0.78

0.51

0.41

0.84

0.65

0.61

Wood or Forest Land thin stand, poor cover, no mulch

good cover

---

---

0.34

---

0.59

0.45

0.70

0.59

Open Spaces, Lawns, Parks,

Golf Courses, Cemeteries

Good Condition

Fair Condition

grass cover on 75% or more

grass cover on 50% to 75%

---

---

0.25

0.45

0.51

0.63

0.65

0.74

Commercial and Business

Area

85% impervious 0.84 0.90 0.93 0.96

Industrial Districts 72% impervious 0.67 0.81 0.88 0.92

Residential

Average Lot Size (acres)

1/8

1/4

1/3

1/2

1

average % impervious

65

38

30

25

20

0.59

0.29

---

---

---

0.76

0.55

0.49

0.45

0.41

0.86

0.70

0.67

0.65

0.63

0.90

0.80

0.78

0.76

0.74

Paved Areas parking lots, roofs, driveways,

etc.

0.99 0.99 0.99 0.99

Streets and Roads paved with curbs & storm sewers

Gravel

dirt

0.99

0.57

0.49

0.99

0.76

0.69

0.99

0.84

0.80

0.99

0.88

0.84

NOTE: Values are based on NRCS (formerly SCS) definitions and are average values.

Source: Technical Manual for Land Use Regulation Program, Bureau of Inland and Coastal

Regulations, Stream Encroachment Permits, New Jersey Department of

Environmental Protection

D. Determination of Rainfall Intensity Rate (I): Determine the Time of

Concentration (Tc) in minutes for the drainage basin. Refer to Section 3.5

for additional information.

Determine the value for rainfall intensity for the selected recurrence interval

with a duration equal to the Time of Concentration from Figures 3-2 through

3-4. Rainfall Intensity "I" curves are presented in Figures 3-2 through 3-4.

The curves provide for variation in rainfall intensity according to location,

storm frequency, and Time of Concentration. Select the curve of a particular

region where the site in question is located (see Figure 3-1 for

determination of the particular region). For project that fall on the line or

Drainage Design

span more than one boundary, the higher intensity should be used for the

entire project. The Regions can be defined by the following:

North Region: All Counties north of the Mercer and Monmouth County

lines.

South Region: All Counties South of the Hunterdon, Somerset, and

Middlesex County lines except for those areas located in the East

Region.

East Region: The eastern region is all municipalities east of the line

delineated by the South municipal boundary of Sea Isle City, Cape May

County to the South and Western boundary of Dennis Township, Cape

May County to the western boundaries of Upper Township, Cape May

County and Estell Manor City, Atlantic County to the West and North

boundary of Weymouth Township, Atlantic County to the North boundary

of Estell Manor City, Atlantic County to the North and East boundary of

Weymouth Township, Atlantic County to the North boundary of Egg

Harbor Township, Atlantic County to the East and North boundary of

Galloway Township, Atlantic County to the North boundary of Port

Republic City, Atlantic County to the East and North boundary of Bass

River Township, Burlington County to the North boundary of Stafford

Township, Ocean County to the East and North boundary of Harvey

Cedars Boro, Ocean County.

The I-D-F curves provided were determined from data from the NOAA Atlas

14, Volume 2, Precipitation-Frequency of the United States. Development of

Intensity-Duration-Frequency (I-D-F) curves is currently available in a

number of computer programs. The programs develop an I-D-F curve based

on user-supplied data or select the data from published data such as Hydro-

35 or the aforementioned NOAA Atlas 14, Volume 2. Appendix A of HEC-12

contains an example of the development of rainfall intensity curves and

equations.

Drainage Design

Figure 3-1

Drainage Design

Figure 3-2

Drainage Design

Figure 3-3

Drainage Design

Figure 3-4

Drainage Design

Use of computer program-generated I-D-F curves shall be accepted

provided the results match those obtained from Figures 3-2 through 3-4.

3.4 US Natural Resources Conservation Service (NRCS) Methodology

Techniques developed by the US Natural Resources Conservation Service

(NRCS), formerly the US Soil Conservation Service (SCS) for calculating rates

of runoff require the same basic data as the Rational Method: drainage area, a

runoff factor, Time of Concentration, and rainfall. The NRCS approach,

however, is more sophisticated in that it considers also the time distribution of

the rainfall, the initial rainfall losses to interception and depression storage, and

an infiltration rate that decreases during the course of a storm. With the NRCS

method, the direct runoff can be calculated for any storm, either real or

fabricated, by subtracting infiltration and other losses from the rainfall to obtain

the precipitation excess. Details of the methodology can be found in the NRCS

National Engineering Handbook, Section 4.

Two types of hydrographs are used in the NRCS procedure, unit hydrographs

and dimensionless hydrographs. A unit hydrograph represents the time

distribution of flow resulting from 1 inch of direct runoff occurring over the

watershed in a specified time. A dimensionless hydrograph represents the

composite of many unit hydrographs. The dimensionless unit hydrograph is

plotted in nondimensional units of time versus time to peak and discharge at

any time versus peak discharge.

Characteristics of the dimensionless hydrograph vary with the size, shape, and

slope of the tributary drainage area. The most significant characteristics

affecting the dimensionless hydrograph shape are the basin lag and the peak

discharge for a specific rainfall. Basin lag is the time from the center of mass of

rainfall excess to the hydrograph peak. Steep slopes, compact shape, and an

efficient drainage network tend to make lag time short and peaks high; flat

slopes, elongated shape, and an inefficient drainage network tend to make lag

time long and peaks low.

The NRCS method is based on a 24-hour storm event which has a certain storm

distribution. The Type III storm distribution should be used for the State of New

Jersey. To use this distribution it is necessary for the user to obtain the 24-hour

rainfall value for the frequency of the design storm desired. The 24-hour rainfall

values for each county in New Jersey can be obtained from the NRCS and are

contained in Table 3-3:

Drainage Design

Table 3-3

New Jersey 24-Hour Rainfall Frequency Data

Rainfall amounts in Inches

County Rainfall Frequency Data

1-Year 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year

Atlantic 2.8 3.3 4.3 5.2 6.5 7.6 8.9

Bergen 2.8 3.3 4.3 5.1 6.3 7.3 8.4

Burlington 2.8 3.4 4.3 5.2 6.4 7.6 8.8

Camden 2.8 3.3 4.3 5.1 6.3 7.3 8.5

Cape May 2.8 3.3 4.2 5.1 6.4 7.5 8.8

Cumberland 2.8 3.3 4.2 5.1 6.4 7.5 8.8

Essex 2.8 3.4 4.4 5.2 6.4 7.5 8.7

Gloucester 2.8 3.3 4.2 5.0 6.2 7.3 8.5

Hudson 2.7 3.3 4.2 5.0 6.2 7.2 8.3

Hunterdon 2.9 3.4 4.3 5.0 6.1 7.0 8.0

Mercer 2.8 3.3 4.2 5.0 6.2 7.2 8.3

Middlesex 2.8 3.3 4.3 5.1 6.4 7.4 8.6

Monmouth 2.9 3.4 4.4 5.2 6.5 7.7 8.9

Morris 3.0 3.5 4.5 5.2 6.3 7.3 8.3

Ocean 3.0 3.4 4.5 5.4 6.7 7.9 9.2

Passaic 3.0 3.5 4.4 5.3 6.5 7.5 8.7

Salem 2.8 3.3 4.2 5.0 6.2 7.3 8.5

Somerset 2.8 3.3 4.3 5.0 6.2 7.2 8.2

Sussex 2.7 3.2 4.0 4.7 5.7 6.6 7.6

Union 2.8 3.4 4.4 5.2 6.4 7.5 8.7

Warren 2.8 3.3 4.2 4.9 5.9 6.8 7.8

Central to the NRCS methodology is the concept of the Curve Number (CN)

which relates to the runoff depth and is itself characteristic of the soil type and

the surface cover. CN’s in Table 2-2 (a to d) of the TR-55 Manual (June 1986)

represent average antecedent runoff condition for urban, cultivated agricultural,

other agricultural, and arid and semiarid rangeland uses. Infiltration rates of

soils vary widely and are affected by subsurface permeability as well as surface

intake rates. Soils are classified into four Hydrologic Soil Groups (A, B, C, and

D) according to their minimum infiltration rate. Appendix A of the TR-55 Manual

defines the four groups and provides a list of most of the soils in the United

States and their group classification. The soils in the area of interest may be

identified from a soil survey report, which can be obtained from the local Soil

Conservation District offices.

Several techniques have been developed and are currently available to

engineers for the estimation of runoff volume and peak discharge using the

NRCS methodology. Some of the more commonly used of these methods are

summarized below:

A. NRCS Technical Release 55 (TR-55): The procedures outlined in this

document are the most widely used for the computation of tormwater

Drainage Design

runoff. This methodology is particularly useful for the comparison of pre-

and post-development runoff rates and consequently for the design of

control structures. There are basically two variations of this technique: the

Tabular Hydrograph method and the Graphical Peak Discharge method.

1. The Tabular Method – This method provides an approximation of the

more complicated NRCS TR-20 method. The procedure divides the

watershed into subareas, completes an outflow hydrograph for each

subarea and then combines and routes these hydrographs to the

watershed outlet. This method is particularly useful for measuring the

effects of changed land use in a part of the watershed. The Tabular

method should not be used when large changes in the curve number

occur among subareas or when runoff flow rates are less than 1345

ft3/s for curve numbers less than 60. However, this method is

sufficient to estimate the effects of urbanization on peak rates of

discharge for most heterogeneous watersheds.

2. Graphical Peak Discharge Method – This method was developed

from hydrograph analysis using TR-20, “Computer Program for Project

Formulation-Hydrology” (NRCS 1983). This method calculates peak

discharge using an assumed hydrograph and a thorough and rapid

evaluation of the soils, slope and surface cover characteristics of the

watershed. The Graphical method provides a determination of peak

discharge only. If a hydrograph is required or subdivision is needed,

the Tabular Hydrograph method should be used. This method should

not be used if the weighted CN is less than 40.

For a more detailed account of these methods and their limitations the

design engineer is referred to the NRCS TR-55 document.

B. US Army Corps of Engineers HEC-1 Model: This model is used to

simulate watershed precipitation runoff processes during flood events. The

model may be used to simulate runoff in a simple single basin watershed or

in a highly complex basin with a virtually unlimited number of sub-basins

and for routing interconnecting reaches. It can also be used to analyze the

impact of changes in land use and detention basins on the downstream

reaches. It can serve as a useful tool in comprehensive river basin planning

and in the development of area-wide watershed management plans. The

NRCS Dimensionless Unitgraph Option in the HEC-1 program shall be used.

Other synthetic unit hydrograph methods available in HEC-1 can be used

with the approval of the Department.

The HEC-1 model is currently supported by a number of software vendors

which have enhanced versions of the original US Army Corps HEC-1 model.

Refer to the available Program Documentation Manual for additional

information.

C. The NRCS TR-20 Model: This computer program is a rainfall-runoff

simulation model which uses a storm hydrograph, runoff curve number and

channel features to determine runoff volumes as well as unit hydrographs to

estimate peak rates of discharge. The dimensionless unit hydrographs from

sub-basins within the watershed can be routed through stream reaches and

impoundments. The TR-20 method may be used to analyze the impact of

development and detention basins on downstream areas. The parameters

Drainage Design

needed in this method include total rainfall, rainfall distribution, curve

numbers, Time of Concentration, travel time and drainage area.

3.5 Time of Concentration (Tc)

The Time of Concentration (Tc) is the time for runoff to travel from the

hydraulically most distant point of the watershed to a point of interest within

the watershed. It may take a few computations at different locations within the

drainage area to determine the most hydraulically distant point. Tc is computed

by summing all the travel times for consecutive components of the drainage

conveyance system.

Tc influences the shape and peak of the runoff hydrograph. Development

usually decreases the Tc, thereby increasing the peak discharge, but Tc can be

increased as a result of (a) ponding behind small or inadequate drainage

systems, including storm drain inlets and road culverts, or (b) reduction of land

slope through grading.

A. Factors Affecting Time of Concentration and Travel Time

1. Surface Roughness: One of the most significant effects of

development on flow velocity is less retardance of flow. That is,

undeveloped areas with very slow and shallow overland flow through

vegetation become modified by development; the flow is then delivered

to streets, gutters, and storm sewers that transport runoff downstream

more rapidly. Travel time through the watershed is generally decreased.

2. Channel Shape and Flow Patterns: In small watersheds, much of the

travel time results from overland flow in upstream areas. Typically,

development reduces overland flow lengths by conveying storm runoff

into a channel as soon as possible. Since channel designs have efficient

hydraulic characteristics, runoff flow velocity increases and travel time

decreases.

3. Slope: Slopes may be increased or decreased by development,

depending on the extent of site grading or the extent to which storm

sewers and street ditches are used in the design of the tormwater

management system. Slope will tend to increase when channels are

straightened and decrease when overland flow is directed through storm

sewers, street gutters, and diversions.

B. Computation of Travel Time and Time of Concentration: Water

moves through a watershed as sheet flow, street/gutter flow, pipe flow,

open channel flow, or some combination of these. Sheet flow is sometimes

commonly referred to as overland flow. The type of flow that occurs is a

function of the conveyance system and is best determined by field

inspection, review of topographic mapping and subsurface drainage plans.

A brief overview of methods to compute travel time for the components of

the conveyance system is presented below.

1. Rational Method: Travel time for each flow regime shall be calculated

as described below:

Drainage Design

a. Sheet Flow: Using the slope and land cover type, determine the

velocity from Figure 3-5. Sheet flow can only be computed for flow

distances of 100 feet or less and for slopes of 24% or less

b. Gutter Flow: The gutter flow component of Time of Concentration

can be computed using the velocity obtained from the Manning

equation for the triangular gutter of a configuration and longitudinal

slope as indicated by roadway geometry.

c. Pipe Flow: Travel time in a storm sewer can be computed using full

flow velocities for the reach as appropriate.

d. Open Channel Flow: Travel time in an open channel such as a

natural stream, swale, man-made ditch, etc., can be computed using

the velocity obtained from the Manning equation or other acceptable

computational procedure for open channel flow such as HEC-2.

Time of concentration (Tc) is the sum of travel time (Tt) values for the

various consecutive flow segments:

Tc = Tt1 + Tt2 + . . . Ttm

where:

Tc = total Time of Concentration

Tt = travel time for each flow segment

m = number of flow segments

2. TR-55: The NRCS TR-55 method separates the flow into three basic

segments: sheet flow, shallow concentrated flow, and open channel. The

maximum length of sheet flow to be used is 150 feet. The open channel

portion may be a natural channel, man-made ditch, or gutter flow along

the roadway. The open channel portion time is determined by using the

Manning’s equation or other acceptable procedure for open channel flow

such as HEC-2. Refer to TR-55, Chapter 3 for detailed information on the

procedures.

The minimum Time of Concentration used shall be 10 minutes.

3.6 Flood Routing

The traditional design of storm drainage systems has been to collect and

convey storm runoff as rapidly as possible to a suitable location where it can be

discharged. This type of design may result in major drainage and flooding

problems downstream. Under favorable conditions, the temporary storage of

some of the storm runoff can decrease downstream flows and often the cost of

the downstream conveyance system. Flood routing shall be used to document

the required storage volume to achieve the desired runoff control.

A hydrograph is required to accomplish the flood routing. A hydrograph

represents a plot of the flow, with respect to time. The predicted peak flow

occurs at the time, Tc. The area under the hydrograph represents the total

volume of runoff from the storm. A hydrograph can be computed using either

the Modified Rational Method (for drainage areas up to 20 acres) or the Soil

Conservation Service 24-hour storm methodology described in previous

sections. The Modified Rational Method is described in detail in Appendix A-5 of

the NJDOT's Soil Erosion and Sediment Control Standards.

Drainage Design

Storage may be concentrated in large basin-wide regional facilities or

distributed throughout the watershed. Storage may be developed in roadway

interchanges, parks and other recreation areas, small lakes, ponds and

depressions. The utility of any storage facility depends on the amount of

storage, its location within the system, and its operational characteristics. An

analysis of such storage facilities should consist of comparing the design flow at

a point or points downstream of the proposed storage site with and without

storage. In addition to the design flow, other flows in excess of the design flow

that might be expected to pass through the storage facility should be included

in the analysis. The design criteria for storage facilities should include:

release rate,

storage and volume,

grading and depth requirements,

outlet works, and

location.

Control structure release rates shall be in accordance with criteria outlined in

Section 2.0, Drainage Policy. Multi-stage control structures may be required to

control runoff from different frequency events.

Storage volume shall be adequate to meet the criteria outlined in Section 2.2,

Stormwater Management and Non-Point Source Pollution Control, to attenuate

the post-development peak discharge rates or Section 2.3 to meet the

allowable water surface elevation.

Outlet works selected for storage facilities typically include a principal spillway

and an emergency overflow, and must be able to accomplish the design

functions of the facility. Outlet works can take the form of combinations of drop

inlets, pipes, weirs, and orifices. Standard acceptable equations such as the

orifice equation (Q = CA(2GH)1/2) or the weir equation (Q = CL(H)3/2) shall be

used to calculate stage-discharge relationships required for flood routings. The

total stage-discharge curve shall take into account the discharge characteristics

of all outlet works. Detailed information on outlet hydraulics can be found in the

"Handbook of Hydraulics", by Brater and King.

Stormwater storage facilities are often referred to as either detention or

retention facilities. For the purposes of this section, detention facilities are

those that are designed to reduce the peak discharge and detain the quantity of

runoff required to achieve this objective for a relatively short period of time.

These facilities are designed to completely drain after the design storm has

passed. Retention facilities are designed to contain a permanent pool of water.

Since most of the design procedures are the same for detention and retention

facilities, the term storage facilities will be used in this chapter to include

detention and retention facilities.

Drainage Design

Figure 3-5

Drainage Design

Routing calculations needed to design storage facilities, although not

extremely complex, are time consuming and repetitive. Many reservoir

routing computer programs, such as HEC-1, TR-20 and Pond-2, are

available to expedite these calculations. Use of programs to perform

routings is encouraged.

Section 11.0 and 12.0 contain standards related to stormwater management

and quality control.

Drainage Design

4.0 Channel Design

4.1 Introduction

Open channels, both natural and artificial, convey flood waters. Natural

channels are crossed at highway sites and often need to be modified to

accommodate the construction of a modern highway. Channels in the form of

roadside ditches are added to the natural drainage pattern.

This part contains design methods and criteria to aid the design engineer in

preparing designs incorporating these factors. Other open channel analysis

methods and erosion protection information is also included.

4.2 Channel Type

The design of a channel is formulated by considering the relationship between

the design discharge, the shape, slope and type of material present in the

channel’s bank and bed. Either grassed channels or non-erodible channels are

typically used. The features of each are presented in the following narrative.

A. Grassed Channels: The grassed channel is protected from erosion by a

turf cover. It is used in highway construction for roadside ditches, medians,

and for channel changes of small watercourses. A grassed channel has the

advantage of being compatible with the natural environment. This type of

channel should be selected for use whenever possible.

B. Non-erodible Channel: A non-erodible channel has a lining that is highly

resistant to erosion. This type of channel is expensive to construct, although

it should have a very low maintenance cost if properly designed. Non-

erodible lining should be used when stability cannot be achieved with a

grass channel. Typical lining materials are discussed in the following

narrative.

1. Concrete Ditch Lining: Concrete ditch lining is extremely resistant to

erosion. Its principal disadvantages are high initial cost, susceptibility to

failure if undermined by scour and the tendency for scour to occur

downstream due to an acceleration of the flow velocity on a steep slope

or in critical locations where erosion would cause extensive damage.

2. Aggregate Ditch Lining: This lining is very effective on mild slopes. It

is constructed by dumping crushed aggregate into a prepared channel

and grading to the desired shape. The advantages are low construction

cost and self-healing characteristics. It has limited application on steep

slopes where the flow will tend to displace the lining material.

3. Alternative Linings: Other types of channel lining such as gabion, or

an articulated block system may be approved by the Department on a

case-by-case basis, especially for steep sloped high velocity

applications. HEC-11, Design of Riprap Revetment provides some design

information on other types of lining.

4.3 Site Application

The design should consider site conditions as described below.

Drainage Design

A. Road Ditches: Road ditches are channels adjacent to the roadway used to

intercept runoff and groundwater occurring from areas within and adjacent

to the right-of-way and to carry this flow to drainage structures or to

natural waterways.

Road ditches should be grassed channels except where non-erodible lining is

warranted. A minimum desirable slope of 0.5% should be used.

B. Interceptor Ditch: Interceptor ditches are located on the natural ground

near the top edge of a cut slope or along the edge of the right-of-way to

intercept runoff from a hillside before it reaches the backslope.

Interceptor ditches should be built back from the top of the cut slope, and

generally at a minimum slope of 0.5% until the water can be emptied into a

natural water course or brought into a road ditch or inlet by means of a

headwall and pipe. In potential slide areas, tormwater should be removed

as rapidly as practicable and the ditch lined if the natural soil is permeable.

C. Channel Changes: Realignment or changes to natural channels should be

held to a minimum. The following examples illustrate conditions that

warrant channel changes:

1. The natural channel crosses the roadway at an extreme skew.

2. The embankment encroaches on the channel.

3. The natural channel has inadequate capacity.

4. The location of the natural channel endangers the highway embankment

or adjacent property.

D. Grade Control Structure: A grade control structure allows a channel to be

carried at a mild grade with a drop occurring through the structure (check

dam).

4.4 Channel Design Procedure

The designed channel must have adequate capacity to convey the design

discharge with 1 foot of freeboard.

Methods to design grass-lined and non-erodible channels are presented in the

following narrative.

A. Grassed Channel: A grassed channel shall have a capacity designated in

Section 2.4 – Recurrence Interval.

A non-erodible channel should be used in locations where the design flow

would cause a grassed channel to erode.

The design of the grassed channel shall be in accordance with the NJDOT

Soil Erosion and Sediment Control Standards Manual.

B. Non-Erodible Channels: Non-erodible channels shall have a capacity as

designated in Section 2.4 – Recurrence Interval. The unlined portion of the

channel banks should have a good stand of grass established so large flows

may be sustained without significant damage.

Drainage Design

The minimum design requirements of non-erodible channels shall be in

accordance with the NJDOT Soil Erosion and Sediment Control Standards

Manual where appropriate unless otherwise stated in this section.

1. Capacity: The required size of the channel can be determined by use of

the Manning’s equation for uniform flow. Manning’s formula gives

reliable results if the channel cross section, roughness, and slope are

fairly constant over a sufficient distance to establish uniform flow. The

Manning’s equation is as follows:

Q =

1.486 AR2/3S1/2

n

where

Q = Flow, cubic feet per second (ft3/s)

n = Manning’s roughness coefficient

Concrete, with surface as indicated: Friction Factor Range

1. Formed, no finish

2. Trowel finish

3. Float finish

4. Float finish, some gravel on bottom

5. Gunite, good section

6. Gunite, wavy section

0.013-0.017

0.012-0.014

0.013-0.015

0.015-0.017

0.016-0.019

0.016-0.022

A = Area, square feet (ft2)

P = Wetted perimeter, feet (ft)

R = Hydraulic radius (A/P)

S = Slope (ft/ft)

Design manuals such as Hydraulic Design Series No. 3 and No. 4 can be

used as a reference for the design of the channels.

For non-uniform flow, a computer program, such as HEC-2, should be

used to design the channel.

2. Height of Lining: The height of the lined channel should be equal to

the normal depth of flow (D) based on the design flow rate, plus 1 foot

for freeboard if possible.

3. Horizontal Alignment: Water tends to superelevate and cross waves

are formed at a bend in a channel. If the flow is supercritical (as it will

usually be for concrete-lined channels), this may cause the flow to erode

the unlined portion of the channel on the outside edge of the bend. This

problem may be alleviated either by superelevating the channel bed,

adding freeboard to the outside edge, or by choosing a larger radius of

curvature. The following equation relates freeboard to velocity, width,

and radius of curvature:

Drainage Design

H =

V2W

32.2Rc

where

H =

V =

W =

Rc =

Freeboard in feet (ft.)

Velocity in ft/s

Bottom width of channel in feet (ft.)

Radius of curvature in feet (ft.)

4. Additional Design Requirements:

a. The minimum d50 stone size shall be 6 inches.

b. The filter layer shall be filter fabric wherever possible.

c. A 3 feet wide by 3 feet deep cutoff wall extending a minimum of 3

feet below the channel bed shall be provided at the upstream and

downstream limits of the non-erodible channel lining.

d. Additional design requirements may be required for permit

conditions or as directed by the Department.

e. Gradation of Aggregate Lining: The American Society of Civil

Engineers Subcommittee recommends the following rules as to the

gradation of the stone:

(1) Stone equal to or larger than the theoretical d50, with a few larger

stones, up to about twice the weight of the theoretical size

tolerated for reasons of economy in the utilization of the quarried

rock, should make up 50 percent of the rock by weight.

(2) If a stone filter blanket is provided, the gradation of the lower 50

percent should be selected to satisfy the filter requirements

between the stone and the upper layer of the filter blanket.

(3) The depth of the stone should accommodate the theoretically

sized stone with a tolerance in surface in rule 1. (This requires

tolerance of about 30 percent of the thickness of the stone.)

(4) Within the preceding limitations, the gradation from largest to

smallest sizes should be quarry run.

C. Water Quality Channel Design: The design of a water quality channel

shall be in accordance with NJDOT and NJDEP requirements. Detailed

requirements regarding water quality control is included in Section 12.0

Water Quality.

Drainage Design

5.0 Drainage of Highway and Pavements

5.1 Introduction

Effective drainage of highway pavements is essential to maintenance of the

service level of highways and to traffic safety. Water on the pavement slows

traffic and contributes to accidents from hydroplaning and loss of visibility from

splash and spray. Free-standing puddles which engage only one side of a

vehicle are perhaps the most hazardous because of the dangerous torque levels

exerted on the vehicle. Thus, the design of the surface drainage system is

particularly important at locations where ponding can occur.

Runoff Collection and Conveyance System Type

Roadway runoff is collected in different ways based on the edge treatment,

either curbed or uncurbed. Runoff collection and conveyance for a curbed

roadway is typically provided by a system of inlets and pipe, respectively.

Runoff from an uncurbed roadway, typically referred to as “an umbrella

section”, proceeds overland away from the roadway in fill sections or to

roadside swales or ditches in roadway cut sections.

Conveyance of surface runoff over grassed overland areas or swales and

ditches allows an opportunity for the removal of contaminants. The ability of

the grass to prevent erosion is a major consideration in the design of grass-

covered facilities. Use of an “umbrella” roadway section may require additional

ROW.

Areas with substantial development adjacent to the roadway, particularly in

urbanized areas, typically are not appropriate for use of a roadway “umbrella”

section.

The decision to use an “umbrella” section requires careful consideration of the

potential problems. Benefits associated with “umbrella” sections include cost

savings and eliminating the possibility of vehicle vaulting. “Umbrella” sections

used on roadways with higher longitudinal slopes have been found to be prone

to berm washouts. Debris build-up along the edge of the roadway creates a

curb effect that prevents sheet flow and directs the water along the edge of the

roadway. This flow usually continues along the edge until a breach is created,

often resulting in substantial erosion. Some situations may also warrant

installing inlets along the edge of an “umbrella” section to pick up water which

may become trapped by berm buildup or when snow is plowed to the side of

the roadway and creates a barrier that will prevent sheet flow from occurring.

Bermed sections are designed with a small earth berm at the edge of the

shoulder to form a gutter for the conveyance of runoff. Care should be taken to

avoid earth berms on steep slopes that would cause erosive velocities yielding

berm erosion.

An “umbrella” section should be used where practical. However, low points at

umbrella sections should have inlets and discharge pipes to convey the runoff

safely to the toe of slope. A Type “E” inlet and minimum 15 inch diameter pipe

shall be used to drain the low point. Snow inlets (see Section 5.12) shall be

provided where the pile up of snow in the berm area prevents drainage of the

low points.

Drainage Design

“Umbrella” sections should be avoided on land service roadways where there

are abutting properties and driveways.

Slope treatment shall be provided at all low points of umbrella sections and all

freeway and interstate projects to provide erosion protection (see NJDOT

Standard Details).

5.3 Types of Inlets Used by NJDOT

Inlet grate types used by NJDOT consist of two types, combination inlets (with

a curb opening), and grate inlets (without a curb opening) as shown on the

current standard details as summarized below:

1. Combination Inlets B, B1, B2, C, D1, D2

2. Grate Inlets A, B Mod., B1 Mod., B2 Mod., E, E1, E2, ES

Inlets Type B1, B2, B1 Modified, B2 Modified, E1 or E2 will be used as

necessary to accommodate large longitudinal pipes. A special inlet shall be

designed, with the appropriate detail provided in the construction plans, and

the item shall be designated "Special Inlet", when the pipe size requires a

structure larger than a Type B2, B2 Modified or E2. A special inlet shall also be

designed, with the appropriate detail provided in the construction plans, and

the item shall be designated "Special Inlet", when the transverse pipe size

requires a structure larger than the standard inlet types.

Drainage structure layout should minimize irregularities in the pavement

surface. Manholes should be avoided where practicable in the traveled way and

shoulder. An example is a widening project where inlets containing a single pipe

should be demolished and the pipe extended to the proposed inlet, as opposed

to placing a slab with a standard manhole cover or square frame with round

cover on the existing inlet and extending the pipe to the new inlet.

5.4 Flow in Gutters (Spread)

The hydraulic capacity of a gutter depends on its cross-section geometry,

longitudinal grade, and roughness. The typical curbed gutter section is a right

triangular shape with the curb forming the vertical leg of the triangle. Design

shall be based on the following frequencies:

Recurrence Interval Facility Description

15-Year Freeway or interstate highway

10-Year Land service highway

The Manning equation has been modified to allow its use in the calculation of

curbed gutter capacity for a triangular shaped gutter. The resulting equation is:

Drainage Design

Q = (0.56/n)(Sx5/3)(So

1/2) T8/3 (1)

where

Q = rate of discharge in ft3/s

n = Manning's coefficient of gutter roughness

(Table 5-1)

Sx = cross slope, in ft/ft

So = longitudinal slope, in ft/ft

T = spread or width of flow in feet

The relationship between depth of flow (y), spread (T), and cross slope (Sx) is

as follows:

Table 5-1

Roughness Coefficients

Manning’s "n"

Street and Expressway Gutters

a. Concrete gutter troweled finish 0.012

b. Asphalt pavement

1) Smooth texture

2) Rough texture

0.013

0.016

c. Concrete gutter with asphalt pavement

1) Smooth

2) Rough

0.013

0.015

d. Concrete pavement

1) Float finish

2) Broom finish

0.014

0.016

e. Brick 0.016

For gutters with small slope where sediment may accumulate,

increase all above values of "n" by 0.002.

5.5 Limits of Spread

The objective in the design of a drainage system for a highway pavement

section is to collect runoff in the gutter and convey it to pavement inlets in a

manner that provides reasonable safety for traffic and pedestrians at a

reasonable cost. As spread from the curb increases, the risks of traffic accidents

and delays and the nuisance and possible hazard to pedestrian traffic increase.

The following shall be used to determine the allowable spread.

1. Width of inside and outside shoulder along interstate and freeway mainline

2. 1/3 width of ramp proper, 1/3 of live lanes next to curb and lanes adjacent

to inside and outside shoulders on land service roads

3. 1/2 width of acceleration or deceleration lanes

The limits of spread are summarized in Table 5-2.

y = TSx, depth in gutter, at deepest point in feet.

Drainage Design

Table 5-2

Limits of Spread

Lane Configuration Interstate and Freeways Land Service Roads

Live Lanes next to

Shoulder

(inside & outside)

Full Shoulder 1/3 Width of Lane

Live Lanes next to

Curb

--- 1/3 Width of Lane

Ramp Proper 1/3 Width of Ramp 1/3 Width of Ramp

Accel/Decel Lanes 1/2 Width of Lane 1/2 Width of Lane

5.6 Inlets

There are separate design standards for grates in pavement or other ground

surfaces, and for curb opening inlets. Each standard is described below. These

standards help prevent certain solids and floatables (e.g., cans, plastic bottles,

wrappers, and other litter) from reaching the surface waters of the State. For

new roadway projects and reconstruction of existing highway, storm drain inlets

must be selected to meet the following design requirements:

A. Grates in Pavement or Other Ground Surfaces

Many grate designs meet the standard. The first option (especially for

storm drain inlets along roads) is simply to use the Department’s bicycle

safe grate. The other option is to use a different grate, as long as each

“clear space” in the grate (each individual opening) is:

• No larger than seven (7.0) square inches; or

• No larger than 0.5 inches (½ inch) across the smallest dimension

(length or width).

B. Curb-Opening Inlets

If the storm drain inlet has a curb opening, the clear space in that curb

opening (or each individual clear space, if the curb opening has two or

more clear spaces) must be:

• No larger than two (2.0) inches across the smallest dimension

(length or width) - many curb opening inlets installed in recent years

meet this criterion; or

• No larger than seven (7.0) square inches

C. Exemptions

The requirements for Grates in Pavement or Other Ground Surfaces or

Curb-Opening Inlets do not apply in certain circumstances. See the New

Jersey Department of Environmental Protection Highway Agency

Stormwater Guidance and “Stormwater Management Rule”, N.J.A.C. 7.8

for a complete list of exemptions.

Storm Drain inlets that are located at rest areas, service areas, maintenance

facilities, and along streets with sidewalks operated by the Department are

required to have a label placed on or adjacent to the inlet. The label must

Drainage Design

contain a cautionary message about dumping pollutants. The message may be

a short phrase and/or graphic approved by the Department. The message may

be a short phrase such as “The Drain is Just for Rain”, “Drains to [Local

Waterbody]”, “No Dumping. Drains to River”, “You Dump it, You Drink it. No

Waste Here”. or it may be a graphic such as a fish. Although a stand-alone

graphic is permissible, the Department strongly recommends that a short

phrase accompany the graphic.

The hydraulic capacity of an inlet depends on its geometry and gutter flow

characteristics. Inlets on grade demonstrate different hydraulic operation than

inlets in a sump. The design procedures for inlets on grade are presented in

Section 5.7, "Capacity of Gutter Inlets on Grade". The design procedures for

inlets in a sump are presented in Section 5.8, "Capacity of Grate Inlets at Low

Points". Proper hydraulic design in accordance with the design criteria

maximizes inlet capture efficiency and spacing. The inlet efficiency should be a

minimum of 75%.

5.7 Capacity of Gutter Inlets on Grade

Collection capacity for gutter inlets on grade shall be determined using the

following empirical equation:

Qi = 16.88y1.54(S0.233/Sx0.276)

where

Qi = flow rate intercepted by the grate (ft3/s)

y = gutter depth (ft) for the approach flow

S = longitudinal pavement slope

Sx = transverse pavement slope

The equation was developed for the standard NJDOT Type “A” grate

configuration and is to be used for all inlet grate types without modification.

An alternative procedure, that yields results reasonably close to those obtained

by using the runoff collection capacity equation presented above, is to compute

the collection capacity in accordance with the procedures presented in Federal

Highway Administration, Hydraulic Engineering Circular No. 12 (HEC-12)

“Drainage of Highway Pavements” using the following parameter values:

Grate type P-1-7/8-4

Constant representative splash-over velocity of 5.77 ft/s

Constant effective grate length of 2.66 feet

All other parameter values for use in this procedure are as stated in HEC-12.

Use of computer programs is encouraged to perform the tedious hydraulic

capacity calculations. HEC-12 contains useful charts and tables. The HEC-12

procedure is also incorporated in a number of computer software programs.

Drainage Design

5.8 Capacity of Grate Inlets at Low Points

Hydraulic evaluation of the bicycle safe grate reveals that the grate functions as

a weir for approach flow depths equal to or less than 9 inches and as an orifice

for greater depths. Procedures to compute the collection capacity for each

condition are presented separately below.

Weir Flow

Collection capacity shall be determined using equation 17 presented on page 69

of HEC-12:

Qi = CwPy1.5

where


Cw = weir coefficient

P = perimeter around the open area of the grate

(as shown on chart 11, on page 71 of HEC-12)

y = depth (ft) for the approach flow

The weir flow coefficient is 3.0. The perimeter around the open area for various

NJDOT bicycle safe grate configurations and the resultant product of CwP are

summarized as follows:

Inlet Type Perimeter* (ft) CwP*

A, B Mod., B1 Mod., B2 Mod. 5.28 15.84

B, B1, B2, C, D1, D2, E 6.96 20.88

ES 5.18 15.54

*Type “B”, “C”, and “D” inlets have a curb opening that

allows runoff to enter the inlet even when debris partly

clogs the grate. The equations must be modified for

use with inlets that do not have a curb opening to

account for reduced interception capacity resulting from

debris collecting on the grate. The perimeter around

the open area of the grate (P) used in the weir

equation should be divided in half for inlets without a

curb opening. The perimeter and resultant product of

CwP for inlet types “A”, “B Mod.”, “E” and “ES” shown in

the table reflect this modification.

Orifice Flow

Collection capacity shall be determined using equation 18 presented on page 69

of HEC-12 (1984):

Drainage Design

Qi = CoAo(2gy)0.5

where


Co = orifice coefficient

Ao = clear opening area of a single grate


g = gravitational acceleration of 32.2 ft/sec2

The orifice flow coefficient is 0.67. The clear opening area and resultant product

of CoAo for various NJDOT bicycle safe grate configurations are summarized as

follows:

Inlet Type Clear Opening Area* (ft2) CoAo*

A, B Mod., B1 Mod., B2 Mod. 1.45 0.97

B, B1, B2, C, D1, D2, E, ES 2.90 1.94

*Type “B” “C”, and “D” inlets have a curb opening that allows

runoff to enter the inlet even when debris partly clogs the grate.

The equations must be modified for use with inlets that do not

have a curb opening to account for reduced interception capacity

resulting from debris collecting on the grate. The clear opening

area of the grate (Ao) used in the orifice equation should be

divided in half for inlets without a curb opening. The clear

opening area and resultant product of CoAo for inlet types” A”, “B

Mod.”, “E” and “ES” reflect this modification.

5.9 Location of Inlets

Proper inlet spacing enhances safety by limiting the spread of water onto the

pavement. Proper hydraulic design in accordance with the design criteria

maximizes inlet capture efficiency and spacing. Inlets should be located

primarily as required by spread computations. See Section 5.7 and Section 5.8.

Additional items to be considered when locating inlets include:

A. Low points in gutter grade. Adjust grades to the maximum extent possible

to ensure that low point do not occur at driveways, handicap accessible

areas, critical access points, etc.

B. At intersections and ramp entrances and exits to limit the flow of water

across roadways.

C. Upgrade of cross slope rollover at the point fifty (50) feet upstream of the

0% cross slope.

D. Upgrade of all bridges and downgrade of bridges in fill section before the

end of curb where the curb is not continuous.

E. Along mainline and ramps as necessary to limit spread of runoff onto

roadway in accordance with Section 5.5.

Drainage Design

5.10 Spacing of Inlets

The spacing of inlets along the mainline and ramps is dependent upon the

allowable spread and the capacity of the inlet type selected. Maximum distance

between inlets is 400 feet. The procedure for spacing of inlets is as follows:

A. Calculate flow and spread in the gutter. Tributary area is from high point to

location of first inlet. This location is selected by the design engineer.

Overland areas that flow toward the roadway are included.

B. Place the first inlet at the location where spread approaches the limit listed

in Section 5.9.

C. Calculate the amount of water intercepted by the inlet, check the grate

efficiency. This efficiency should be a minimum of 75%.

D. The water that bypasses the first inlet should be included in the flow and

spread calculation for the next inlet.

E. This procedure is repeated to the end of the system. Sample calculations

are presented in Section 13.0.

5.11 Depressed Gutter Inlet

Placing the inlet grate below the normal level of the gutter increases the cross-

flow towards the opening, thereby increasing the inlet capacity. Also, the

downstream transition out of the depression causes backwater which further

increases the amount of water captured.

A. Locations of Depressed Inlets

1. All inlets in shoulders greater than 4 feet wide.

2. All inlets in one-lane, low speed ramps.

3. Inlets will not be depressed next to a riding lane, acceleration lane,

deceleration lane, two-lane ramps, and direct connection ramps or

within the confines of a bridge approach and transition slab.

B. Limits of Depression

1. Begin depression a distance of 4 feet upgrade of inlet.

2. End depression a distance of 2 feet downgrade of inlet.

3. Begin depression 4 feet out from gutter line.

4. Depth of depression, 2 inches below projected gutter grade.

See NJDOT Standard Roadway Construction / Traffic Control / Bridge

Construction Details; CD-603-3, Method of Depressing Inlets at Shoulders.

C. Spacing of Depressed Inlets

Use the same procedure as described in Section 5.9. This method will give a

conservative distance between inlets; however, this will provide an added

safety factor and reduce the number of times that water will flow on the

highway riding lanes when the design storm is exceeded.

Drainage Design

5.12 Snow Melt Control

Roadway safety can be enhanced by snow melt runoff control. Collection of

snow melt runoff is important on the high side of superelevated roadways and

at low points. A discussion of each situation and the design approach is outlined

below.

A. Snowmelt Collection on High Side of Superelevation

Collection of snow melt on the high side of a superelevated section from

roadway and berm areas before it crosses the roadway prevents icing

during the freeze-thaw process. Therefore, a safety offset or small shoulder

(4 feet wide) sloped back towards the curb at a rate of 6% will provide a

means to convey the snow melt water to inlets installed for this purpose.

The snow melt inlets should be placed along the outer curbline at the

upstream side of all intersections and at convenient cross drain locations.

The snow melt inlets should be connected to the drainage system with a 15

inch diameter pipe to the trunk storm sewer. The small shoulder and snow

inlets will not be designed to control stormwater runoff but shall be

designed to handle only the small amount of expected flow from the

snowmelt.

B. Snowmelt Collection at Low Points

Collection of snowmelt is important at low points where the pile-up of snow

over existing inlets prevents draining of snowmelt and runoff off the edge of

road. The addition of inlets placed away from the edge of curb and beyond

anticipated snow piles provides a means to drain snowmelt.

Snow inlets are required at all roadway profile low points. All snow inlets

shall be Type "E". Snow inlets shall not be depressed.

Snow inlets shall be provided in the shoulder immediately adjacent to the

travel lane without encroaching on the travel lane.

Snow inlets shall not be installed in shoulders where the width is so narrow

that placement of a snow inlet will encroach upon the inlet at the curb.

Pipes draining snow inlets shall be a minimum 15 inches diameter, sloped at

a 1% minimum grade wherever possible.

5.13 Alternative Runoff Collection Systems

Standard roadway inlets are used to collect runoff on curbed roadways.

Compliance with the established spread criteria for roadways with flat grades

typically requires many inlets, usually installed at close intervals. Use of

alternative collection systems such as trench drains may be appropriate to

reduce the number of inlets required to satisfy the spread criteria. Therefore,

use of trench drains for runoff collection on roads with flat grades may be

warranted. The trench drain should be located upstream of the inlet to which it

connects. The length of trench drain should provide the capture capacity that

together with the inlet limits bypass at the inlet to zero.

Drainage Design

Trench drain capture computations require consideration of both frontal and

side flow capture. Frontal flow captured by the narrow trench drain is small and

is, therefore, disregarded. Side flow into the trench drain is similar to flow into

a curb opening inlet. Hydraulic evaluation procedures for curb opening inlets

are described in FHWA HEC-12. Side flow is computed using the procedures for

curb opening inlets presented in FHWA HEC-12. The trench drain must be long

enough to intercept the bypass after frontal flow plus the additional runoff

contributed by the roadway for the length of the trench drain. The process

includes the following steps:

A. Compute the total runoff to the inlet.

B. Compute the frontal flow captured by inlet with no bypass allowed for the

spread limited to the width of the grate. The runoff to be intercepted by the

trench drain is the total runoff minus the runoff captured by the inlet.

C. Compute the length of trench drain required to capture the discharge using

the curb opening inlet procedures in FHWA HEC-12. The computed length

shall be multiplied by two to reflect inefficiencies due to clogging.

Maintenance requirements for trench drains should also be considered in the

evaluation of trench drains. Use of a trench drain system should be discussed

with the Department early in the design process with recommendations

submitted prior to completion of the Initial Submission.

Drainage Design

6.0 Storm Drains

6.1 Introduction

A storm drain is that portion of the roadway drainage system that receives

runoff from inlets and conveys the runoff to some point where it can be

discharged into a ditch, channel, stream, pond, lake, or pipe. This section

contains the criteria and procedures for the design of roadway drainage

systems.

6.2 Criteria for Storm Drains

Storm drains shall be designed using the following criteria where applicable:

A. Minimum pipe size is 15 inches.

B. Minimum pipe size is 18 inches downstream of mainline lowpoints.

C. Storm sewer pipe materials for proposed systems typically include concrete,

corrugated metal, aluminum alloy and Smooth interior High Density

Polyethylene (HDPE). Manning's roughness coefficient "n" for concrete and

HDPE pipe is .012. Manning's roughness coefficient values for corrugated

metal and aluminum alloy pipe are presented in Table 6-1. Manning's

roughness coefficients for other materials occasionally encountered are

indicated below:

Table 6-1.

Manning's roughness coefficients - Other

Manning's Roughness Coefficient, "n"

Closed Culverts:

Vitrified clay pipe 0.012-0.014

Cast-iron pipe, uncoated 0.013

Steel pipe 0.009-0.011

Brick 0.014-0.017

Monolithic concrete:

1. Wood forms, rough 0.015-0.017

2. Wood forms, smooth 0.012-0.014

3. Steel forms 0.012-0.013

Cemented rubble masonry walls:

1. Concrete floor and top 0.017-0.022

2. Natural floor 0.019-0.025

Laminated treated wood 0.015-0.017

Vitrified clay liner plates 0.015

D. Design to flow full, based on uniform flow.

E. Minimum self-cleaning velocity of 2.5 ft/sec. should be maintained wherever

possible.

Drainage Design

Table 6-2

Values of Coefficient of Manning's Roughness (n)

for Corrugated Metal and Aluminum Alloy Pipe

(Unpaved Inverts and Unlined Pipe)

Annular

2 2/3" x 1/2"

Corrugations

Helical Corrugations*

All

Diameters

1 1/2" x 1/4" 2 2/3" x 1/2"

8

inch

10

inch

12

inch

18

inch

24

inch

36

inch

48

inch

60

inch

& Larger

0.024 0.012 0.014 0.011 0.013 0.015 0.018 0.020 0.021

Annular

3" x 1"

Helical - 3" x 1"

48

inch

54

inch

60

inch

66

inch

72

inch

78

inch

& Larger

0.027 0.023 0.023 0.024 0.025 0.026 0.027

Annular

5" x 1"

Helical - 5" x 1"

54

inch

60

inch

66

inch

72

inch

78

inch

& Larger

0.025 0.022 0.023 0.024 0.025 0.027

*The "n" values shown above for helical corrugations apply only when spiral

flow can be developed. The design engineer must assure himself/herself that

spiral flow will occur in his/her design situation. Spiral flow will not occur

when the following conditions exist, in which case the "n" value for annular

corrugations is to be used:

1. Partly full flow

2. Non-circular pipes, such as pipe arches

3. When helical C.M.P. is lined or partly lined

4. Short runs less than 20 diameters long

Pipe arches have the same roughness characteristics as their equivalent

round pipes

F. Structural design (class or gauge) of storm drains shall be in accordance

with current AASHTO Standard Specifications for Highway Bridges.

Structural evaluation of storm drains may be made using the following

texts/references where appropriate if they are consistent with AASHTO:

1. Concrete: Concrete Pipe Design Manual American Concrete Pipe

Association

2. Corrugated Metal Pipe: Handbook of Steel Drainage and Highway

Construction Products

3. Aluminum Alloy Pipes (as recommended by manufacturer)

4. Smooth interior HDPE (as recommended by manufacturer)

G. Maximum grade on which concrete pipe should be placed is 10%.

H. HDPE, shall be used for pipe lengths outside of the roadbed only. HDPE pipe

is not allowed for lateral pipes or for the outlet pipe to a receiving

watercourse or water body. The density of polyethylene pipe is less than

Drainage Design

water, therefore when wet conditions are expected, polyethylene pipe will

float and should not be specified. End sections for HDPE shall be concrete.

I. Flared end-sections should be used whenever and wherever possible, for

concrete and metal pipe.

J. Pipe sizes should not decrease in the downstream direction even though an

increase in slope would allow a smaller size.

K. Pipe slopes should conform to the original ground slope so far as possible to

minimize excavation.

L. For durability, the minimum thickness for steel pipe is 14 gauge and for

aluminum alloy pipe is 16 gauge. In extremely corrosive areas and where

high abrasion can be expected the design engineer shall determine whether

heavier gauges should be used.

M. Material types: See Figure 6-1.

a. Concrete, HDPE, and Aluminum Alloy pipe may be used in the shaded

area.

b. Concrete, HDPE, Steel and Aluminum Alloy pipe may be used in the

unshaded area.

N. Alternate Items:

a. When the estimated cost of the pipes is more than $50,000, alternate

bid items are required.

b. Alternate pipe materials include corrugated metal, aluminum alloy, and

HDPE.

c. Some materials may be eliminated as alternate items due to unstable

support, high impact, concentrated loading, limited clearance, steep

gradients, etc.

O. The drainage layout should attempt to avoid conflicts with existing

underground utilities and such items as utility poles, signal pole

foundations, guide rail posts, etc. Implementation of the following design

approaches may be necessary.

a. Use of pipe material with the lowest friction factor to minimize pipe size

b. Use of elliptical or arch pipe to minimize vertical dimension of pipe.

c. Test pits should be obtained early in the design process to obtain

horizontal and vertical information for existing utilities. If the suggested

design approaches do not avoid conflict, use of special drainage

structures may be used to avoid the utility.

Drainage Design

Figure 6-1

Drainage Design

When alternate bid pipe materials are required, separate hydraulic calculations

must be developed and submitted for each material considered (concrete,

corrugated metal/aluminum alloy) using the respective roughness coefficients.

The reason for exclusive use of a pipe material must be explained in the

Drainage Report.

P. Round corrugated metal pipe shall have helical corrugations, except that

annular corrugated pipe may be used where velocity reduction is desired.

Q. Drainage structures must accommodate all pipe materials used including

concrete, corrugated metal, aluminum alloy, and HDPE.

R. Aluminum alloy pipe shall not be used as a section or extension of a steel

pipe.

S. Precast manholes or inlets shall not be used for pipes 54 inches or larger

diameter or when three or more pipes tie in and at least two of them are

connected at some angles. When these conditions exist, cast-in-place inlets

or manholes are more practical.

T. Cleaning existing drainage pipes and structures shall be incorporated on all

projects when the existing drainage system has substantial accumulation of

sediments. The cleaning shall extend to the first structure beyond the

project limits.

U. On projects where contaminated areas have been identified, the drainage

system should be designed to avoid these locations, if possible. If avoidance

is not feasible, a completely watertight conveyance system, including

structures such as manholes, inlets, and junction chambers, should be

designed to prevent contaminated groundwater or other pollutants from

entering the system. Possible methods to accomplish this include joining

pipe sections with a watertight sealant and/or gaskets, or the use of welded

steel pipe. Retrofitting existing pipes to make them watertight may require

installation of an appropriate internal liner. The design engineer shall

provide recommendations prior to proceeding with the final design.

V. The soffits (overts) between the inflow and outflow pipes at a drainage

structure shall be matched where possible. A minimum 1 inch drop between

inverts within the structure shall be provided, if feasible.

W. Existing drainage facilities that are not to be incorporated into the proposed

drainage system are to be completely removed if they are in conflict with

any element of the proposed construction. Existing drainage facilities that

are not to be incorporated into the proposed drainage system that do not

conflict with any element of the proposed construction are to be abandoned.

Abandonment of existing drainage facilities requires the following:

1. Plugging the ends of the concrete pipes to remain. Metal pipes shall be

either removed or filled.

2. Filling abandoned pipes in accordance with geotechnical

recommendations.

3. Removing the top of the drainage structure to 1 foot below the bottom

of the pavement box, breaking the floor of the structure, and filling the

structure with either granular material or concrete in accordance with

geotechnical recommendations.

Y. A concrete collar, as shown in the standard detail CD-602-1.3, will be used

to join existing to proposed pipe of similar materials unless an approved

adapter fitting is available.

Drainage Design

6.3 Storm Sewer Design

Hydraulic design of the drainage system is performed after the locations of

inlets, storm drain layout, and outfall discharge points have been determined.

Hydraulic design of the drainage pipe is a two step process. The first step

establishes the preliminary pipe size based on hydrology and simplified

hydraulic computations. The second step is the computation of the hydraulic

grade line (HGL) for the system. This step refines the preliminary pipe size

based on calculation of the hydraulic losses in the system using the hydrology

computed in the first step for each section of pipe. The procedures to be

performed in step 1 are presented in Section 6.4, "Preliminary Pipe Size". The

procedures to be performed in step 2 are presented in Section 6.5, "Hydraulic

Grade Line Computations".

6.4 Preliminary Pipe Size

The preliminary design proceeds from the upstream end of the system toward

the outlet at which the system connects to the receiving downstream system.

The design runoff for each section of pipe is computed by the Rational formula

using the total area that contributes runoff to the system and the Time of

Concentration to the upstream end of the pipe. The Time of Concentration

increases in the downstream direction of the design and the rainfall intensity

consequently decreases. All runoff from the contributing area is assumed to be

captured. The inlet capture and by-pass computations used to determine the

inlet layout are not used in the hydraulic computation.

The preliminary storm drain size should be computed based on the assumption

that the pipe will flow full or practically full for the design runoff. The Manning

equation should be used to compute the required pipe size. This preliminary

procedure determines the required pipe size based on the friction losses in the

pipe. All other losses are disregarded in the preliminary design. In general, the

longitudinal grade of the roadway over the pipe being designed should be used

as the slope in the hydraulic computation where practical. The HGL

computations, as explained in Section 6.5, consider all losses and establish the

actual pipe size required.

Figure 6-2 is recommended for use as guidance in performing the preliminary

drainage system design. Use of computer programs to perform the

computations is encouraged. The computational procedures and output results

and presentation format presented in the FHWA Hydrain-Hydra program are

recommended for use. Use of other computer programs is acceptable provided,

as a minimum, the computational procedures and presentation of output are

similar to those presented in Figure 6-2.

The following is an explanation of the Preliminary Storm Drain Computation

Form, Figure 6-2. Data is to be presented for each reach of pipe being

designed. The numbers refer to each column in Figure 6-2.

Drainage Design

PRELIMINARY STORM

DRAIN COMPUTATION FORM

FIGURE 6-2

Computed: Date: Route:

Section:

Checked: Date: County:

Station and Offset

(1)

L (ft)

Drainage Area

“A”

(Acres)

Runoff

Coef-ficient

“C”

“A” x “C” Flow Time

“Tc”

(min.)

Rainfall

“I”

in/hr

Total

Chapter 2 R

unoff

Q=CIA

ft3/S

Dia.

Pipe

ft

Slope

ft/ft

Capacity

Full

ft3/S

Velocity

ft/s

Invert Elevation

Junction

From

Junction

To

Incre-

ment

Total

Incre-

ment

Total

Overland

To

Inlet

In

U/S

Pipe

Cum.

Total in

Pipe*

Flowing

Full

Design

Flow

U/S

End

D/S

End

(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

For Time of Concentration, use larger of overland flow to inlet or cumulative time in pipe.

Drainage Design

1. Station and Offset

Input the location of the upstream and downstream structure for each pipe

reach being designed referenced from the base line, survey line, or profile

grade line (PGL) shown on the construction documents.

2. Length in feet

Input the distance between the centerline of the upstream and downstream

structure.

3. Incremental Drainage Area in acres

Input the drainage area to each structure for each area with a different

runoff coefficient that contributes runoff to the upstream structure.

4. Total Drainage Area in acres

Input the cumulative total drainage area. This is a running total of column 3.

5. Runoff Coefficient

Input the rational method runoff coefficient for each area contributing runoff

to the structure.

6. Incremental “A” x “C”

Input the incremental drainage area times its runoff coefficient for each

area contributing runoff to the structure.

7. Total “A” x “C”

Input the cumulative drainage area times the runoff coefficient. This is a

running total of column 6.

8. Flow Time (Time of Concentration) to Inlet in Minutes

Input the overland Time of Concentration to each structure.

9. Flow Time in Pipe in Minutes

Input the flow time in the pipe upstream of the upstream junction (junction

from). This time is computed by dividing the pipe length by the actual

design flow velocity in the pipe (Column #2 divided by Column #17) for the

pipe section upstream of the junction from structure (Column #1). The first

pipe length will have no value. The flow time in the pipe will be used to

compute the cumulative Time of Concentration (travel time) in the pipe.

10. Cumulative Time in the Pipe in Minutes

Input the cumulative time in the pipe. This is a running total of column 9. If

the overland flow to the inlet is greater than the cumulative time in the

pipe, then that overland flow time will be added to subsequent flow time in

the pipe to determine the longest cumulative Time of Concentration.

11. Rainfall Intensity “I” in inches per Hour

Input the rainfall intensity using Figures 3-1 through 3-4 and the longest

Time of Concentration. The longest Time of Concentration is determined by

using the larger of the overland flow time to the inlet (column 8) or the

cumulative time in the pipe (column 10).

12. Total Runoff (Q = CIA) in cubic feet per Second

Compute the total runoff using the area, runoff coefficient, and rainfall

intensity identified in step 11.

13. Pipe Diameter in feet

Compute the required pipe diameter using Manning’s equation based on full

flow. The tailwater is assumed to be at the elevation of the pipe soffit.

14. Slope in feet per feet

Input the pipe slope used for the pipe design. The slope is typically as close

as possible to the roadway longitudinal grade over the pipe reach being

designed.

15. Capacity in cubic feet per Second

Compute the pipe capacity using the Manning’s equation and full flow

conditions.

Drainage Design

16. Velocity (full) in feet per Second

Compute the pipe velocity using the full pipe capacity (V = Q/A).

17. Velocity (design) in feet per Second

Compute the pipe velocity using the design discharge.

18. Invert Elevation (Upstream End)

Input the pipe invert elevation at the upstream end.

19. Invert Elevation (Downstream End)

Input the pipe invert elevation at the downstream end.

6.5 Hydraulic Grade Line computations

The Hydraulic Grade Line (HGL) should be computed to determine the water

surface elevation throughout the drainage system for the design condition. The

HGL is a line coinciding with either (1) the level of flowing water at any point

along an open channel, or (2) the level to which water would rise in a vertical

tube connected at any point along a pipe or closed conduit flowing under

pressure. The HGL is normally computed at all junctions, such as inlets and

manholes. All head losses in the storm drainage system are considered in the

computation. The computed HGL for the design runoff must remain at least 1

foot below the top of grate or rim elevation.

Hydraulic control, also commonly referred to as "tailwater", is the water surface

elevation from which the HGL calculations are begun. "Tailwater" elevation is

established by determining water surface elevation at the locations where the

new drainage system will discharge to the receiving waterway, such as a

stream, ditch, channel, pond, lake, or an existing or proposed storm sewer

system. The tailwater selected for the design should be the water surface

elevation in the receiving waterway at the Time of Concentration for the

connecting roadway storm sewer being designed or analyzed.

When the system is under pressure and when a higher level of accuracy is

required considering storage in the pipe system, pressure flow routing can be

performed using computer programs such as the "Pressure Flow Simulation"

option in the FHWA Hydrain-Hydra program. Use of a pressure flow routing in

the design of a new drainage system or analysis of an existing drainage system

should be evaluated early in the initial design. A pressure flow routing is

typically appropriate only in special cases, primarily when the available storage

attenuates the peak discharge to the extent that downstream pipe sizes are

minimized.

Figures 6-3 and 6-4 are recommended for use as guidance in performing HGL

computations. HGL line computations must be provided for all projects. Use of

computer software acceptable to the Department to perform the computational

procedures is encouraged. The computational procedures, output results, and

presentation format similar to what is presented in Figures 6-3 and 6-4 are

required as a minimum.

The following is an explanation of the computation of the Hydraulic Grade Line

using Figure 6-3. The computed hydraulic grade line (HGL) for the design runoff

must remain at least 1 foot below the roadway finished grade elevation at the

drainage structure. Data is to be presented for each reach of pipe being

designed. The pipe designation presented in the explanation refers to the pipe

being designed unless otherwise noted. The numbers refer to each column in

Figure 6-3.

Drainage Design


Input the location of the upstream and downstream structure for each pipe

reach being designed, referenced from the base line, survey line, or profile

grade line (PGL) where applicable from the construction documents.

2. Pipe Diameter (Ø) in feet

Input downstream pipe diameter.

3. Flow (Q) in cubic feet per Second

Input flow in downstream pipe (outflow pipe).

4. Pipe velocity in feet per Second

Input the design velocity of the pipe.

5. Hydraulic Radius (R) in feet

Input the hydraulic radius (area divided by wetted perimeter) of the pipe.

6. Length (L) of Pipe in feet

Input the distance between the centerline of the upstream and downstream

structure.

7. Manning's "n" Roughness Coefficient

Input the Manning's coefficient "n". Use 0.012 for concrete and smooth

interior plastic pipe. The Manning's “n” values for corrugated metal and

aluminum alloy pipe are shown in Table 6-2.

8. Velocity Head (h) in feet

Compute the velocity head, h = V2/2g, Where g = acceleration due to

gravity.

9. Friction Loss (Hf) in feet

Compute the friction loss in the pipe using the equation:

Hf =

29.14n2L

R1.33

X

V2

2g

HYDRAULIC GRADE LINE

COMPUTATION FORM

FIGURE 6-3


Section:


Station & Offset

(1)

(2)

Q

(3)

V

(4)

R

(5)

L

(6)

n

(7)

h

(8)

Hf

(9)

He

(10)

Hi

(11)

Hs

(12)

Ht

(13)

TW

(14)

HGL

(15)

TOS

(16)

CL

(17)

Junction

From

Junction

To

Dia.

ft

Flow

ft3/S

Vel.

ft/s

Hydraulic

radius

ft

Length

ft

Manning'

s

Vel.

Head

ft

Fric.

Loss

ft

Exit

Loss

ft

Entr.

Loss

ft

Struct.

Loss*

ft

Total

Head

Loss

ft

Tail-

water

Elev.

ft

Head-

water

Elev.

ft

Top of

Struct.

Elev.

ft

TOS-

HGL

ft

h = Velocity head, = (V)2/2g Hi = Entrance Loss = Ki(V)2/2g Refer to Table 6-3 for values of Ki

Hf = Friction Loss, = 29.14N2L X (V)2 He = Exit Loss, He = (V)2/2g

R1.33 2g

* For structural (junction) losses in inlets, manholes, see Figure 6-3.

STRUCTURAL AND BEND

LOSS COMPUTATION FORM

FIGURE 6-4


Section:


(1)

(2)

Q

(3)

v

(4)

v2

2g

(5)

(6)

(7)

Ks

(8)

Hs

(9)

A

(10)

Kb

(11)

Hb

(12)

Hs + Hb

(13)

Junction

Station &

Offset

Downstream

Dia.

ft

Downstream

Flow

ft3/S

Downstream

Velocity

ft/s

Velocity

Head

ft

Junction

Type

(L,N or O)

Flow

Type

(P or O)

Structural

Loss

Coeff.

Structural

Loss

ft

Angle

deg.

Bend

Factor

Bend Loss

ft

Structural

Loss +

Bend Loss

ft

Hs = Structural Loss = KS X (V)2 , Ks from Table 6-4 NOTES: 1) Junction Type 2) Flow Type

2g L = with Lateral P = Pressure N = with No Lateral O = Open Channel Hb = Bend Loss = Kb X (V)2 , Kb from Figure 7-1 O = with Opposed Laterals 2g

Drainage Design

Table 6-3

Entrance Loss Coefficients (Ki)

This table shows values of the coefficient Ki to apply to the velocity head V2/2g to

determine the loss of head at the entrance of a structure such as a culvert or

conduit, operating full or partly full with control at the outlet.

Entrance head loss Hi = Ki V2/2g

Type of Structure and Design of Entrance Coefficient, Ki

A. Concrete Pipe

Projecting from fill, socket end (groove-end) 0.2

Projecting from fill, square cut end 0.5

Headwall or headwall and wingwalls

Socket end of pipe (groove-end) 0.2

Square-edge 0.5

Rounded (radius = D/12) 0.2

Mitered to conform to fill slope 0.7

End-section conforming to fill slope * 0.5

Beveled edges, 33.7° or 45° bevels 0.2

Side or slope-tapered inlet 0.2

B. CMP or CMPA

Projecting from fill (no headwalls) 0.9

Headwall or headwall and wingwalls

Square-edge 0.5

Mitered to conform to fill slope 0.7

End-section conforming to fill slope * 0.5

C. Concrete Box

Headwall parallel to embankment (no wingwalls)

Square-edged on 3 edges 0.5

Rounded on 3 edges to radius of 1/12 barrel dimension,

Or beveled edges on 3 sides 0.2

Wingwalls at 30 - 75 degrees to barrel

Square-edged at crown 0.4

Crown edge rounded to radius of 1/12 barrel dimension,

Or beveled top edge 0.2

Wingwalls at 10 - 25 degrees to barrel


Wingwalls parallel (extension of sides)


*NOTE: "End sections conforming to fill slope”, made of either metal or concrete,

are the sections commonly available from manufacturers. From limited hydraulic

tests they are equivalent in operation to a headwall in both inlet and outlet control.

10. Exit Loss (He) in feet

Compute the exit loss of the drainage system using the equation:

He = V2/2g, Where V = velocity of outflow pipe

The exit loss is computed where the drainage system discharges to a swale,

stream, pond, etc. via a headwall or a pipe open end. This loss is calculated

for the last downstream pipe segment at the outlet end of the pipe being

designed.

Drainage Design

11. Entrance Loss (Hi) in feet

Compute the entrance loss of the drainage system using the equation:

Hi = KiV2/2g,

Where

Ki = Entrance Loss Coefficient

The entrance loss is computed at the upstream end of the system where the

flow enters the first structure. This is either at a headwall/ end section or

the pipe in the beginning upstream inlet. Entrance loss coefficients are

presented in Table 6-3.

12. Structural Loss (Hs) in feet

Input the structural loss from Figure 6-4. The structural loss corresponds to

the structure at the upstream end of the pipe segment or "junction from".

13. Total Head Loss (Ht) in feet

Compute the total head loss by adding the exit, entrance, friction, and

structural loss. The exit and entrance losses are only added at the beginning

and end of the pipe system, respectively.

14. Tailwater Elevation (TW) in feet

Input the tailwater elevation at the downstream end of the pipe segment

being designed. For the last downstream pipe segment, the tailwater

elevation is established by determining the water surface elevation at the

location where the pipe discharges to a stream, ditch, channel, pond, lake,

or an existing or proposed storm sewer system. The tailwater selected for

the design should be the water surface elevation in the receiving waterway

at the Time of Concentration for the connecting roadway storm sewer being

designed or analyzed. The tailwater elevation for each upstream pipe

segment will be the computed headwater elevation (HGL) for the

downstream pipe segment.

15. Headwater Elevation (HGL) in feet

Compute the HGL at the upstream end of the pipe segment by adding the

total head loss (Ht) to the tailwater elevation (TW) at the downstream end

of the pipe.

16. Top of Structure (TOS) Elevation in feet

Input the top of structure elevation which is the top of grate for inlets and

rim elevation for manholes.

17. Clearance (CL) in feet

Compute the clearance or difference in elevation between the top of

structure (TOS) and the headwater elevation (HGL). The HGL shall be a

minimum of 1 foot below the TOS.

The following is an explanation of the computation of structural losses using

Figure 6-4. Data is to be presented for each reach of pipe being designed. The

numbers refer to each column in Figure 6-4.


Input the location of each drainage structure referenced from the base line,

survey line, or profile grade line (PGL) where applicable from the

construction documents.

2. Pipe Diameter (Ø) in feet

Input downstream pipe diameter (outflow). Equivalent diameter for elliptical

or arch pipes may be used.

3. Flow (Q) in cubic feet per second

Input flow in downstream pipe (outflow pipe).

4. Downstream Velocity (v) in feet per second

Drainage Design

Input the velocity in the pipe.

5. Velocity Head (h) in feet

Compute the velocity head, h=V2/2g

6. Structure Lateral Configuration

The structural loss coefficient is related to the structure lateral configuration

and type of flow. The lateral configuration designation is as follows:

L = Junction with lateral

N = Junction with no lateral

O = Junction with opposed laterals

7. Flow Type

The structural loss coefficient is related to the structure lateral configuration

and type of flow. The flow type designation is as follows:

P = Pressure flow

O = Open channel flow

8. Structural Head Loss Coefficient

The structural head loss coefficient is related to the structure lateral

configuration and type of flow. Insert the coefficient selected from Table 6-4:

Table 6-4

Structure Head Loss Coefficient (Ks)

Flow Condition Lateral Configuration Coefficient

Open Channel 90° Lateral 0.2

Open Channel No Lateral 0.0

Open Channel Opposed 0.2

Pressure 90° Lateral 1.0

Pressure No Lateral 0.3

Pressure Opposed 1.0

Proper application of the structural loss to the drainage system requires an

understanding of which pipe(s) is (are) considered the lateral(s) and which

pipes are considered the main. For simplicity, the inflow pipe with the

majority of the flow entering the structure is considered the main. All other

inflow pipes are considered laterals.

The hydraulic grade line computation for each lateral begins with the water

surface elevation for the junction, which includes the structural head loss

and bend head loss for the structure. No other losses are associated with

the connection of the lateral to the junction.

9. Structural Loss in feet

Compute the structural loss as the product of the structural loss coefficient

(column 8) and velocity head (column 5).

10. Angle (A) in degrees

Input the deflection angle between the inflow and outflow main pipes. The

angle should be between 0 and 90 degrees.

11. Bend Factor

Insert bend factor from Figure 7-1.

12. Bend Loss in feet

Drainage Design

Compute the bend loss as the product of the bend factor (column 11) and

velocity head (column 5).

13. Structural Loss + Bend Loss in feet

Compute the sum of the structural loss (column 9) and the bend loss

(column 12).

Drainage Design

7.0 Median Drainage

7.1 Introduction

The basic purpose of a median is to separate opposing lanes of traffic. The

widths, grade and shape of a median is determined for the most part by safety

considerations. A wide, shallow, depressed median is usually selected as best

fulfilling the median purpose.

A provision to drain the median by means of inlets must be included in the

median design. Median inlets shall be provided to limit the depth of flow to 6

inches to confine the spread to the median and below the pavement subgrade.

This section contains procedures and criteria for the design of median drainage.

7.2 Median Inlet Type

All median inlets are to be Type "E".

7.3 Median Design Criteria - Continuous Grade

Median inlets should intercept the total design flow from its discharge area plus

any by-pass from upstream. The drainage area to each inlet must be adjusted

by inlet spacing to limit the design flow to a maximum depth of 6 inches.

Because of the variable parameters in the spread calculations, each inlet must

be investigated.

A. The recurrence interval used in the design is the same as that of the

longitudinal roadway system.

7.4 Procedure for Spacing Median Drains

Channel capacity shall be computed using the procedures presented in Section

4.0, Channel Design.

Inlet capture for inlets on grade shall be computed using the weir equation

stated as follows:

Qi = CwPy1.5

where

Qi = flow rate intercepted by the grate ft3/s

Cw = weir coefficient

P = weir length (ft)


Drainage Design

Figure 7-1

Drainage Design

The weir flow coefficient is 3.0. The weir length to be used is the frontal flow

length of the inlet.

Inlet capture for inlets at low points shall be computed using the procedures in

Section 5.8 "Capacity of Grate Inlets at Low Points".

Judgment should be used in a cut section to place these inlets economically as

well as functionally. Some leeway is afforded the design engineer to place the

median inlets opposite roadway edge inlets. This simplifies connections and

reduces pipe lengths. The water that bypasses the inlet because of the above,

should be added to the next inlet's design runoff.

Drainage Design

8.0 Culvert Design

8.1 Introduction

A highway embankment constitutes a barrier to the flow of water where the

highway crosses water courses. A culvert is a closed conduit that provides a

means of carrying the flow of water through the embankment.

8.2 Culvert Types

A. Pipes: Metal and reinforced concrete pipe culverts are shop manufactured

products available in a range of sizes in the standard shapes. Metal pipes

(aluminum and steel) are available in round and arch shapes. Reinforced

Concrete pipes are available in round and elliptical shapes. Round shapes

are generally more economical, due to their greater strength.

Pipe flow characteristics for different pipes change due to their relative

roughness.

Additional capacity can be obtained with multiple pipe installations. Multiple

installations are accomplished by installing several individual culvert pipes

parallel to each other with enough separation to allow for proper

compaction.

B. Reinforced Concrete Boxes (RCB's): Box culverts are either precast off-

site or constructed in the field by forming and pouring. Box culverts may be

constructed to any desired size in either square or rectangular shapes.

These designs may be easily altered to allow for site conditions. The flow

characteristics of RCB's are very good as their barrels provide smooth flow

and their inlet may be designed for extra efficiency where needed.

Where a multiple culvert installation is indicated, the RCB may be

constructed with two or more barrels. Stream Encroachment Permit

requirements may dictate when multiple culverts can be used. The

minimum width, if possible, will be 10 feet per box. For streams with a

drainage area greater than 50 acres, the Stream Encroachment Permit

requirements will also dictate the need to provide a fish passage in at least

one box culvert. Guidance regarding fish passage provisions in culverts are

presented in Section 8.8.

8.3 Culvert Location

The alignment of a culvert in both plan and profile should ensure efficient

hydraulic performance, as well as keep the potential for erosion and

sedimentation to a minimum. The criteria given in Section 4.0, "Channel

Design”, should be considered in the location of the culvert. Usually, the ideal

location for the culvert is the existing channel, with the slope the same as the

existing channel.

8.4 Culvert Selection

Select a culvert type and size that is compatible with hydraulic performance,

structural integrity and economics. The structural requirements for various

pipes may be found in references (1), (2), and (3).

Drainage Design

8.5 Culvert Hydraulics

Laboratory tests and field observations show two major types of culvert flow:

flow with inlet control and flow with outlet control. Different factors and

formulas are used to compute the hydraulic capacity of a culvert for each type

of control. Under inlet control, the cross-sectional area of the culvert barrel, the

inlet geometry and the amount of headwater or ponding at the entrance are of

primary importance. Outlet control involves the additional consideration of the

elevation of the tailwater in the outlet channel and the slope, roughness and

length of the culvert barrel.

It is possible by involved hydraulic computations to determine the probable

type of flow under which a culvert will operate for a given set of conditions. The

need for making these computations may be avoided, however, by computing

headwater depths from available charts and/or computer programs for both

inlet control and outlet control and then using the higher value to indicate the

type of control and to determine the headwater depth. This method of

determining the type of control is accurate except for a few cases where the

headwater is approximately the same for both types of control. Refer to FHWA

HDS-5 - Hydraulic Design of Highway Culverts for detailed culvert design

procedures.

8.6 Culvert End Structures

Culvert end structures may be used for the following purposes:

A. To improve the hydraulic efficiency of the culvert.

B. To provide erosion protection and prevent flotation.

C. To retain the fill adjacent to the culvert.

These structures include headwalls, concrete flared end sections, corrugated

metal end sections, and improved inlet structures to increase capacity. Each

type is described in the following narrative.

A. Headwall: A headwall is a retaining wall attached to the end of a culvert.

(see current Standard Construction Details CD-610-1). The alignment of the

headwall should be normal to the centerline of the barrel to direct the flow

into the barrel. The wingwalls should be long enough to prevent spillage of

the embankment into the channel. A cutoff wall attached to the downstream

end of the unit if a concrete apron is not provided at the headwall. The

cutoff wall may be a concrete unit across the entire width of the

downstream end of the flared end section. The cutoff wall shall be a

minimum of 1.5 feet thick and 3.0 feet deep (see current Standard

Construction Details).

B. Concrete Flared End Sections: A concrete flared end section is a precast

unit with a beveled and flared end that provides an apron at the outlet end

of the pipe. The bevel approximately conforms to embankment slope.

Limited grading of the embankment is usually required around the end of

the flared end section. Installation of a flared end section requires

installation of a cutoff wall attached to the downstream end of the unit. The

cutoff wall may be a concrete unit across the entire width of the

downstream end of the flared end section. The cutoff wall shall be a

Drainage Design

minimum of 1.5 feet thick and 3.0 feet deep (see current Standard

Construction Details).

C. Corrugated Metal End Sections: A corrugated metal end section is a

beveled and flared end that provides an apron at the outlet end of the pipe.

The bevel approximately conforms to embankment slope. Limited grading of

the embankment is usually required around the end of the end section.

Installation of an end section requires installation of a cutoff wall attached

to the downstream end of the unit. The cutoff wall may be a concrete unit

across the entire width of the downstream end of the section. The cutoff

wall shall be a minimum of 1.5 feet thick and 3.0 feet deep (see current

Standard Construction Details).

D. Improved Inlet: An improved culvert inlet incorporates inlet geometry

refinements to increase the capacity of a culvert operating with inlet control.

These geometry improvements include beveled edges, side tapers and slope

tapers functioning either individually or in combination.

8.7 Flood Routing at Culverts

The presence of substantial storage volume below the allowable headwater

elevation at the upstream end of a culvert warrants evaluation of the resultant

peak flow attenuation. The reduced peak discharge resulting from attenuation

yields a reduced culvert size for a new crossing. Attenuation of the peak

discharge at existing crossings may indicate that the existing culvert is

adequate or may reduce the size of the relief or replacement culvert. For this

reason, flood routing computations shall be performed for all culvert locations

except where the proposed topography indicates that limited storage volume,

such as is typical with deep incised channels, is available.

Flood routing evaluation at a culvert provides a realistic indication of hydrologic

conditions at the culvert entrance. A more realistic assessment can be made

where environmental concerns are important. The extent and duration of

temporary upstream ponding determined by the flood routing computations can

help improve the environmental assessment of the proposed construction.

The design procedure for flood routing through a culvert is the same as for

reservoir routing. Additional information on flood routing and storage is

included in Section 3.6.

8.8 Fish Passage

Fish passage is historically a concern with culverts. Failure to consider fish

passage may block or impede upstream fish movements in the following ways:

Outlet of the culvert is installed above the streambed elevation to where

fish may not be able to enter.

Scour lowers the streambed downstream of the culvert outfall and the

resulting dropoff creates a potential vertical barrier.

High outlet velocity may provide a barrier.

Higher uniform velocities within the culvert than occur in the natural

channel may prevent fish from entering or transiting the culvert.

Abrupt drawdown, turbulence, and accelerated flow at the inlet to the

culvert entrance may prevent fish from exiting the culvert.

Natural channel replaced by an artificial channel may have no zones of

quiescent water in which fish can rest.

Drainage Design

Debris barriers (including ice) upstream or within the culvert may stop fish

movement.

Shallow depths within the culvert during minimum flow periods may

preclude fish passage.

The design engineer is encouraged to refer to the NJDEP Technical Manual for

Land Use Regulation Program, Bureau of Inland and Coastal Regulations,

Stream Encroachment Permits, in addition to Figure 8-1 for the latest

acceptable methods for providing fish passage in all proposed box culvert

installations. For more guidance on fish passage provisions in proposed culvert

installations, contact the NJDEP Division of Fish and Wildlife.

Drainage Design

Figure 8-1

Drainage Design

9.0 Conduit Outlet Protection

The purpose of conduit outlet protection is to provide a stable section of area in

which the exit velocity from the pipe is reduced to a velocity consistent with the

stable condition downstream. The need for conduit outlet protection shall be

evaluated at any location where drainage discharges to the ground surface or a

channel, ditch or stream. This may occur at the downstream end of culverts or

other drainage systems.

The need for conduit outlet protection shall be determined by comparing the

allowable velocity for the soil onto which the pipe discharges to the velocity

exiting the pipe. The allowable velocity for the soil shall be that given in the

NJDOT Soil Erosion and Sediment Control Standards Manual. The velocity in the

pipe shall be that which occurs during passage of the design storm or of the 25-

year storm, whichever is greater. When the velocity in the pipe exceeds the

allowable velocity for the soil, outlet protection will be required.

For a detail of conduit outlet protection for a flared end section or headwall, see

the Standard Roadway Construction Detail CD-602-1.4, "Stormwater Outfall

Protection".

9.1 Riprap Size and Apron Dimensions

Conduit outlet protection and apron dimensions shall be designed in accordance

with procedures in the NJDOT Soil Erosion and Sediment Control Standards

Manual. The minimum d50 stone size shall be 6 inches. A tail water depth equal

to 0.2 Do shall be used where there is no defined downstream channel or where

Tw cannot be computed.

9.2 Energy Dissipators

Energy dissipators are typically required when the outlet velocity is 15 ft/s or

greater. Energy dissipators shall be provided when the stable velocity of the

existing channel is exceeded, or when design of standard riprap conduit outlet

or channel protection results in an impractical stone size and/or thickness.

Energy dissipators for channel flow have been investigated in the laboratory,

and many have been constructed, especially in irrigation channels. Designs for

highway use have been developed and constructed at culvert outlets. All energy

dissipators add to the cost of a culvert; therefore, they should be used only to

prevent or to correct a serious erosion problem that cannot be corrected by

normal design of standard soil erosion and sediment control elements.

The judgment of engineers is required to determine the need for energy

dissipators at culvert outlets. As an aid in evaluating this need, culvert outlet

velocities should be computed. These computed velocities can be compared

with outlet velocities of alternate culvert designs, existing culverts in the area,

or the natural stream velocities. In many streams the maximum velocity in the

main channel is considerably higher than the mean velocity for the whole

channel cross section. Culvert outlet velocities should be compared with

maximum stream velocities in determining the need for channel protection. A

change in size of culvert does not change outlet velocities appreciably in most

cases.

Drainage Design

Outlet velocities for culverts flowing with inlet control may be approximated by

computing the mean velocity for the culvert cross section using Manning's

equation.

Since the depth of flow is not known, the use of tables or charts is

recommended in solving this equation. The outlet velocity as computed by this

method will usually be high because the normal depth, assumed in using

Manning's equation, is seldom reached in the relatively short length of the

average culvert. Also, the shape of the outlet channel, including aprons and

wingwalls, has much to do with changing the velocity occurring at the end of

the culvert barrel. Tailwater is not considered effective in reducing outlet

velocities for most inlet control conditions.

In outlet control, the average outlet velocity will be the discharge divided by the

cross-sectional area of flow at the outlets. This flow area can be either that

corresponding to critical depth, tailwater depth (if below the top of the culvert)

or the full cross section of the culvert barrel.

Additional design information for energy dissipators is included in FHWA HEC-

14, Hydraulic Design of Energy Dissipators for Culverts and Channels.

Drainage Design

10.0 Reset Castings - Manholes and Inlets

10.1 Reset Castings and Construction Practices

Where a manhole or inlet is to be raised using the item, Reset Castings and the

existing hardware is excessively worn or in otherwise poor condition, a new

frame and cover or grate shall be used.

The condition of the existing hardware and its probable performance after

resetting needs to be assessed. If wear has caused the cover to be depressed

more than 1/4 inch below the top of the frame, a new frame and cover or grate

shall be specified.

On new pavement elevations exceeding 3 1/2 inches, castings shall be reset as

follows: on multi-course resurfacing projects, the base and/or binder course

shall be placed before a manhole frame is raised. This increases the accuracy in

bringing the manhole to the proper grade and cross slope and leaves no more

than 1 1/2 inches of casting exposed to traffic, thus permitting the roadway to

be opened to traffic. If the specified cross slope of the overlay is different from

that of the existing pavement, an extension ring with the necessary slope

change built into the casting shall be specified.

For purposes of plan preparation, Cast Iron Extension Frames for Inlets and

Extension Rings for Manholes shall be used to raise existing castings a

maximum of 3 1/2 inches. When existing castings are required to be raised

more than 3 1/2 inches to a maximum of 12 inches, the item Reset Castings

shall be used. The item Reset Castings shall also be used to lower grades and

elevations up to 12 inches. Adjustments of grades and elevations in excess of

12 inches will be considered as reconstructing inlets and manholes and the

appropriate pay items shall be used.

Before Cast Iron Extension Frames or Rings are called for at a particular

location, a determination shall be made by the design engineer as to whether

the existing casting was previously raised using a Cast Iron Extension Frame or

Ring, and what height was used. If a Cast Iron Extension Frame or Ring was

previously used and the sum of the previous resetting plus the proposed

resetting exceeds 3 1/2 inches, then the item Reset Castings or the appropriate

reconstruction item shall be used.

10.2 Extension Rings and Frames

When structures contain existing frames or rings, these extension frames or

rings shall be removed. Multiple extension frames and rings are not allowed.

The design engineer may decide to reset a particular head by either using the

item, Reset Castings, or by installing an extension frame. This decision will

primarily be influenced by the following factors:

A. The height to which the head is to be raised.

B. The maximum height of the casting above the roadway surface when open

to traffic.

C. The prevailing traffic speed and volume.

D. The location of the casting in the traveled way or shoulder.

E. Expected interference with traffic flow.

Drainage Design

F. The actual condition of the casting.

G. The comparative costs of resetting a casting (e.g. in concrete pavement,

resetting is generally more expensive).

While some case-by-case analyses of these factors will be required, if the rise

of head is between 1 1/2 inches to 3 1/2 inches, an extension unit will generally

be specified. If the rise of the elevation is less than 1 1/2 inches or more than 3

1/2 inches, the casting will be reset by the conventional method.

10.3 Extension Rings - Manholes

On all resurfacing projects where the proposed overlay thickness is between 1

1/2 inches and 3 1/2 inches, an extension ring shall be used to reset heads.

When installing the extension ring, any rise above 1 1/2 inches must be paved

over and reset before the surface course is placed unless the binder course is

placed before opening the roadway to traffic.

The minimum thickness for a manhole extension ring is 1 1/2 inches. Since the

Standard Manhole Cover is 2 inches thick, any height adjustments in the range

of 1 1/2 inches and 2 1/4 inches will require a new Heavy Duty Cover (1 inch

thick). Any salvageable cover in good condition can only be used in an

extension ring 2 1/2 inch or more in height.

The following guidelines shall assist in determining where to use Extension

Rings for Existing Manholes:

A. If the rise, R, is from 1 1/2 inches to less than 2 1/2 inches, an Extension

Ring for Heavy Duty Cover (1 inch thick cover) is warranted.

B. If R is 2 1/2 inches to 3 1/2 inches, use a new Extension Ring for Standard

Cover (2 inches thick cover).

C. If R is less than 1 1/2 inches or greater than 3 1/2 inches, use the item

Reset Castings, to raise the manhole.

10.4 Extension Frames - Inlets

The minimum height of an inlet extension frame is 1 3/4 inches. Depending on

how extensively depressed or "dished" an existing inlet may be, an extension of

2 inches, 2 1/2 inches, or 3 inches high may be required to enable the top

elevation of the head to be set flush with the finished grade of a 1 1/2 inches

overlay.

The following guidelines shall assist in determining where to use Extension

Frames for Existing Inlets:

A. If R is 1 3/4 inches to 3 1/2 inches, inclusive, use an extension frame.

B. If R is less than 1 3/4 inches or greater than 3 1/2 inches, the manhole is to

be raised using the item, Reset Castings.

C. In general, inlets use a standard 1 1/4 inches grate on all extension frames.

10.5 Ramping

Ramping around the reset heads prior to final paving shall be accomplished as

follows:

Drainage Design

A. On single course (1 1/2 inches and variable) projects, a circular ramp of hot

mix shall be placed about the periphery of the manhole to extend 3 feet

laterally and shall leave 1/2 inch of the extension ring exposed; this should

avoid the occurrence of under-compacted, shoddy-appearing areas (due to

feathering) when the surface course is placed.

B. For multi-course resurfacing projects, the base and/or binder course should

be placed before the casting is reset. This increases the accuracy of raising

the casting to be flush with the finished pavement and enables the work

progress to be in greater conformity with the policy of not having more than

1 1/2 inches exposed for more than 48 hours.

C. For a 3 inch resurfacing where 1 1/2 inches is to be milled off, after milling,

the bituminous ramp will be placed as for the single course in "A". The

binder course will then be placed so that the casting will end up being set

flush with the finished pavement grade.

D. For the occasional 2 inch overlays, ramps will be constructed as for the 1

1/2 inches course.

E. Do not reset the casting until the topmost (if more than one) bottom course

has been placed so that not more than 1 1/2 inches will be exposed for

more than 48 hours before bringing the pavement to grade.

F. The brickwork shall be set with a high early strength, non-shrink mortar

developing a one-hour compressive strength of 2500 PSI at 70°F. The

mortar should not contain any gypsum, iron particles or chlorides.

Drainage Design

11.0 Stormwater Management

11.1 Introduction

As previously stated in Sections 1.0 and 2.0, stormwater management is an

important consideration in the design of roadway drainage systems.

Stormwater management practices, when properly selected, designed, and

implemented, can be utilized to mitigate the adverse hydrologic and hydraulic

impacts caused by NJDOT facilities and mitigate the loss in groundwater,

thereby protecting the health of streams and wetlands, and the yield of water

supply wells, and downstream areas from increased flooding, erosion, and

water quality degradation. Stormwater management is required if the proposed

roadway project disturbs one (1) or more acres of land or creates at least 0.25

acre of new or additional impervious surface.

This section will focus on design elements of structural stormwater

management facilities common to proposed roadway projects, or retrofits to

existing roadways, which typically include detention basins, infiltration basins,

or a combination thereof. Detention basins may be either wet or dry ponds.

Additional guidance regarding the design of stormwater management facilities

is presented in the Stormwater Best Management Practices Manual. All designs

must comply with the appropriate regulatory requirements and the Stormwater

Best Management Practices Manual.

A. Stormwater Quantity Requirements

As per the NJDEP Stormwater Rules at N.J.A.C. 7.8-5.4(a)3, Stormwater

BMPs shall be designed to one of the following:

1. The post-construction hydrograph for the 2-year, 10-year, and

100-year storm events do not exceed, at any point in time, the

pre-construction runoff hydrographs for the same storm events.

2. There shall be no increase, as compared to the pre-construction

condition, in peak runoff rates of stormwater leaving the project

site for the 2-year, 10-year, and 100-year storm events and that

the increased volume or change in timing of stormwater runoff will

not increase flood damage at or downstream of the site. This

analysis shall include the analysis of impacts of exiting land uses

and projected land uses assuming full development under existing

zoning and land use ordinances in the drainage area.

3. The post-construction peak runoff rates for the 2-year, 10-year,

and 100-year storm events are 50%, 75%, and 80%, respectively,

of the pre-construction rates. The percentages apply only to the

post-construction stormwater runoff that is attributed to the

portion of the site on which the proposed development or project is

to be constructed.

4. In tidal flood hazard areas, stormwater runoff quantity analysis

shall only be applied if the increased volume of stormwater runoff

could increase flood damages below the point of discharge.

Drainage Design

B. Groundwater Recharge Requirements

As per the NJDEP Stormwater Rules at N.J.A.C. 7.8-5.4(a)2, stormwater

BMPs must be designed to perform to the following:

The stormwater BMPs maintain 100% of the average annual

preconstruction groundwater recharge volume for the site; or

The increase in stormwater runoff volume from pre-construction to post-

construction for the 2-year storm is infiltrated. NJDEP has provided an

Excel Spreadsheet to determine the project sites annual groundwater

recharge amounts in both pre- and post-development site conditions. A

full explanation of the spreadsheet and its use can be found in Chapter 6

of the New Jersey Stormwater Best Management Practices Manual. A

copy of the spreadsheet can be downloaded from

http://www.njstormwater.org.

11.2 Methodology

As previously stated in Sections 1.0 and 2.0, specific stormwater management

requirements to control the rate and/or volume of runoff may be dictated by

various regulatory agencies. Groundwater recharge is required by the

Stormwater Management Rule. Peak runoff discharge rates may also be limited

by capacity constraints of existing downstream drainage systems.

The tasks that typically need to be performed in the design of stormwater

management facilities for stormwater quantity and groundwater recharge are

summarized as follows:

A. Detention Basin

calculate inflow hydrographs;

calculate maximum allowable peak outflow rates;

calculate stage vs. storage data for the basin;

calculate stage vs. discharge curve for the outlet; and

perform flood routing calculations.

B. Infiltration Basin

Same as for detention basin except that the stage vs. discharge curve is

based on the infiltration rate; and

The basin must be designed so that the design runoff volume is

completely infiltrated within 72 hours of the end of the storm.

C. Detention/Infiltration Basin

Same as detention basin with the following modifications:

The infiltration rate is typically very small relative to the discharges from

the outlet structure, and is, therefore, disregarded in the stage vs.

discharge curve; and

The basin must be designed so that the volume to be infiltrated is

completely infiltrated within 72 hours of the end of the storm.

Inflow hydrographs shall be computed using either the Modified Rational

Method or the SCS 24-hour storm methodology as described in Section 3.6,

Drainage Design

depending upon the contributory drainage area. The Modified Rational

Method is described in detail in Appendix A-5 of the NJDOT's Soil Erosion

and Sediment Control Standards.

The allowable peak outflow rates shall be determined as follows:

A. For regulated stormwater management facilities, i.e. requiring

regulatory agency review, maximum allowable outflow rates shall be as

dictated by said regulatory agency.

B. For nonregulated stormwater management facilities, i.e. NOT requiring

regulatory agency review, the allowable outflow rate shall avoid an

unreasonable increase in runoff resulting from the project. The peak

outflow rate shall be determined for the roadway design storm and the

storms with a recurrence interval of once in 2-, 10-, and 25-years.

Downstream stability shall be evaluated for any proposed peak outflow

rate that results in an unreasonable increase in the existing peak flow

rate and appropriate action shall be taken to avoid unreasonable erosion

or flooding resulting from the proposed construction.

Storage volume and outlet structure rating curve data are site specific and

will vary for each pond; however, sufficient storage volume shall be

provided and the outlet structures shall be configured so that outflow

requirements as described in Section 11.2 are satisfied.

Flood routing calculations shall be based upon the Storage Indication

Method (Modified Puls). As stated in Section 3.6 the use of computer

software programs such as Pond-2, HEC-1, and/or TR-20 to perform these

iterative routing calculations is encouraged. Any one of these procedures is

acceptable.

A typical method to maintain the existing groundwater recharge is to provide a

retention/extended detention basin or sand and vegetative filter strips. An

analysis of the pre- and post- developed on-site groundwater recharge

conditions can be determined by using the NJDEP’s New Jersey Groundwater

Recharge Spreadsheet found in the New Jersey Best Management Practices

Manual. For Groundwater Recharge, it is important that the permeability rate

be tested at the location of the BMP. The BMP must have a minimum

Permeability of 0.2 to 0.5 inches per hour and the BMP structure must drain in

less than 72 hours. For more guidance on the design of Groundwater Recharge

BMPs, see Chapter 6, of the New Jersey Stormwater Best Management

Practices Manual. Chapter 6 also has guidance on the use of the Groundwater

Recharge Spreadsheet Program. A copy of the Spreadsheet is located in Figures

11-1 and 11-2. The Spreadsheet can be downloaded from

http://www.njstormwater.org.

11.3 Stormwater Management Facility Locations

The location of stormwater management facilities will depend on several factors

such as location of receiving water course, location of roadway profile low

points, groundwater elevations, etc.

The design engineer should first consider, and make maximum use of locations

within NJDOT right-of-way, eg. at interchanges, ramp infield areas, wide

http://www.njstormwater.org/

Drainage Design

medians, before locating facilities which require additional right-of-way.

However, site/project specific constraints will ultimately dictate exact locations

of stormwater management facilities.

11.4 Stormwater Management Facility Design Features

Detention ponds may be excavated depressions (cut) or diked (dammed) by

means of an embankment. It should be noted that any embankment/pond that

raises the water level more than 5 feet above the usual mean, low water

height, or existing ground, when measured from the downstream toe-of-dam to

the spillway crest on a permanent or temporary basis must conform to NJAC

7:20 "Dam Safety Standards", effective May 2, 1995.

Detention ponds shall incorporate the following design features:

A. Pond side slopes shall be 1 (vertical) on 3 (horizontal) or flatter to facilitate

mowing.

B. A low flow channel shall be provided having a minimum slope of 0.5% and

side slopes of 1 on 3 or flatter.

C. The pond bottom shall be graded to drain to the low flow channel at a

minimum slope of 1.0%.

D. A ten (10) foot wide flat safety bench shall be provided 1 foot above the

normal pool elevation in a wet pond.

E. All ponds shall be evaluated for fencing needs. The evaluation shall be

submitted to the Bureau of Landscape Design and Scoping and Review for

their review.

F. To the maximum extent practicable, outlet structures shall be designed so

as to require minimal maintenance. Trash racks and safety grating shall be

provided.

G. Dry detention ponds and the portion of a wet pond above the normal pool

elevation shall be topsoiled and seeded. The Landscape and Urban Design

Unit should be contacted for guidance regarding seeding requirements and

additional landscaping features in and around proposed ponds.

H. The height and fluctuation of the groundwater table shall be taken into

account when designing any wet or dry pond. Design of a dry pond below

the seasonal high water table may result in periodic flooding of the pond.

In addition, an access ramp to the stormwater management facility may be

provided to allow NJDOT maintenance personnel and equipment to enter the

facility for maintenance/cleaning operations. Where an access ramp into

stormwater management facilities for truck access to basin bottom and outlet

structure for maintenance is required, the following criteria should be applied:

Width: 13 feet wide; and

8% slope desirable, 12% maximum.

Refer to the NJDEP Stormwater Best Management Practices Manual for

recommended outlet structure designs and more detailed design data for

stormwater management facilities.

Drainage Design

Figure 11-1

Drainage Design

Figure 11-2

Drainage Design

11.5 Stormwater Management Facility Maintenance

The design engineer shall prepare a Stormwater Management Facility

Maintenance Plan in accordance with the New Jersey Stormwater Rule. At a

minimum, the maintenance plan shall include specific preventative maintenance

tasks and schedules. The maintenance plan shall include at a minimum the

manufacturer’s recommendation on the maintenance of their facility.

Maintenance plan guidelines are available in the New Jersey Stormwater Best

Management Practices Manual. Additional maintenance information is also

provided in the NJDEP Stormwater Management Facility Maintenance Manual,

including recommended maintenance tasks and equipment, inspection

procedures and schedules, ownership responsibilities, and design

recommendations to minimize the overall need for maintenance while

facilitating inspection and maintenance tasks.

A copy of The Stormwater Management Facility Maintenance Plan shall be

submitted to the Division of Maintenance and Operations for review. If NJDEP

permits are required, the Stormwater Management Facility Maintenance Plan

shall be submitted, prior to the submission of the plan to the NJDEP with the

permit application(s). Upon approval of the NJDEP Permit(s), a copy of the

approved permit documentation shall be provided to the Division of

Maintenance and Operations.

Drainage Design

12.0 Water Quality

12.1 Introduction

Stormwater runoff from NJDOT facilities and activities can be a potential

contributor to water quality degradation of receiving waterbodies. This section

will focus on the design of water quality facilities to treat runoff from roadways.

Refer to the NJDEP Stormwater Best Management Practices Handbook and the

NJDOT Soil Erosion and Sediment Control Standards Manual for water quality

measures and recommendations which can be used for other NJDOT facilities

and activities.

Stormwater BMPs shall be designed to reduce the post-construction load of TSS

in stormwater runoff generated from the water quality storm by 80% of the

anticipated load from the developed site. Section 12.0 and the Stormwater Best

Management Practices Manual provide guidance in the planning and design of

these facilities.

For those waters designated in the tables in N.J.A.C. 7:9B-4.15(c) through (h)

for the purposes of implementing the Antidegradation Polices in N.J.A.C. 7:9B-

4, projects involving a Category One waterbody shall be designed such that a

300-foot special water resource protection area is provided on each side of the

waterbody. Encroachment within this 300-foot buffer is prohibited except in

instances where preexisting disturbance exists. Where preexisting disturbance

exists, encroachment is allowed, provided that the 95% TSS removal standard

is met and the loss of function is addressed.

12.2 Methodology

The water quality design storm peak rate and volume shall be determined in

accordance with N.J.A.C. 7:13-2.8(b)2 (PDF Format - Page 23) which currently

states using either of the following:

A. One year, 24-hour storm using SCS Type III rainfall distribution; or

B. 1 1/4 inch of rainfall falling uniformly in two hours.

12.3 Water Quality Treatment Facilities and Design

As indicated in Section 1.1 water quality is an important consideration in

roadway drainage system design. Water quality facilities should be designed in

accordance with all the regulatory requirements that apply.

Examples of water quality measures include, but are not limited to:

Extended dry detention ponds

Wet ponds

Vegetated or biofilter swales

Constructed wetlands

Infiltration basins/trenches

Oil/water separators

Manufactured Water Quality Treatment Devices

Drainage Design

Additional guidance regarding the design of water quality facilities is presented

in the New Jersey Stormwater Best Management Practices Manual and the

following web site: http://www.njstormwater.org.

This section focuses on design elements of those water quality measures most

applicable to roadway projects, i.e. extended dry detention ponds, wet ponds,

vegetated/biofilter swales and, manufactured water quality treatment devices.

Where stormwater management facilities are proposed for roadway projects,

provisions for water quality treatment should be incorporated in the facility

where possible.

For example, stormwater management facilities typically contain a low level

outlet for water quality storm treatment. Stormwater management for the

higher intensity storms (2-year, 10-year, and 100-year) is subsequently

provided above the level of the water quality storm. Note: the term “extended”

indicates that the detention pond is also designed for water quality treatment.

When a detention pond is used to provide water quality treatment, the following

requirements must be met:

A. Beginning at the time of peak storage within the pond, no more than 90%

of the total storm volume shall be released over a 24-hour period; the rate

of release shall be as uniform as possible;

B. The minimum outlet diameter, width or height is 3 inches. If this minimum

outlet size does not provide for the detention times required in A above,

then alternative or additional techniques for the removal of total suspended

solids(TSS) shall be provided; and

C. The species of native and/or non-intrusive exotic vegetation used in the

pond is approved by the Landscape and Urban Design Unit and, if required,

regulatory agencies.

When treatment within a pond is not feasible, the use of vegetated or biofilter

swales is permissible provided that:

A. The water velocity does not exceed 2 feet per second (fps) to allow for

settlement of TSS during the water quality design storm;

B. The slope of the swale shall not be less than 0.5 percent and the length of

the swale shall be of sufficient length to allow for settlement of TSS, taking

into consideration the velocity, depth of flow, and expected loading of TSS,

a minimum length of 300 feet should be used for swales;

C. The residence time, i.e. time within the swale, should be maximized as

much as possible, with five minutes used as the absolute minimum;

D. The design flow depth in mowed swales shall not exceed 3 inches for the

water quality design storm. In swales with wetlands vegetation, the depth

should be at least 1 ½ inches below the height of the shortest species;

E. Trapezoidal swale bottom widths should be no less than 2 feet and side

slopes should be no steeper than 2 horizontal to 1 vertical;

F. Given the above constraints, biofilters should be designed using Manning’s

Equation. Recommended values of Manning’s “n” are 0.020 for grass

biofilters regularly mowed and those with herbaceous wetland plants, and

0.024 for infrequently mowed swales, unless other information is available.

http://www/

Drainage Design

G. If the longitudinal slope of the swale is less than 2 percent or the water

table can reach the root zone of vegetation, water-resistant vegetation shall

be used to survive potential standing water conditions;

H. Vegetation shall be used in the swale to filter out the TSS and to provide a

secondary treatment by absorption of pollutants leached into the soil.

Vegetation used in the swale shall be approved by the Landscape and Urban

Design Unit and, if required, regulatory agencies; and

I. Vegetated swales should not be used as the only method of water quality

treatment below the final discharge of the tormwater drainage system

unless there is no other feasible method of providing water quality

treatment within the project area.

When other water quality measures are not feasible, the use of Manufactured

Water Quality Treatment Devices are permissible. Use of Low Impact

Development techniques should be utilized to the maximum extent possible.

For projects that are subject to the NJDEP Stormwater Management

Regulations, the design engineer must complete the Low Impact Development

Checklist found in the New Jersey Stormwater Best Management Practices

Manual. If the use of a Manufactured Water Quality Treatment Device is

necessary to meet the minimum water quality standards, the manufactured

device should be designed in accordance with the following guidelines:

A. Use of Manufactured Water Quality Treatment Devices are limited to devices

approved by the New Jersey Department of Environmental Protection

(NJDEP). A Complete list of Certified Stormwater Technologies approved by

the NJDEP can be found at http://www.njstormwater.org. Table 12-1 is a

list of devices approved by the NJDEP:

Table 12-1

Approved Manufactured Water Quality Treatment Devices

Product* Manufacturer

TSS %

Removal

Stormwater Management Inc Stormfilter Stormwater Management, Inc. 80%

Vortechnics Stormwater Treatment System Vortechnics Inc. 50%

High Efficiency Continuous Deflective Separator Unit CDS Technologies 50%

Stormceptor Stormwater Treatment System Stormceptor Group of Companies 50%

Bay Saver Separator Device Bay Saver Technologies, Inc. 50%

*The above list represents only those treatment devices currently certified by NJDEP as of May 2005, and should not be interpreted as exhaustive, nor as an endorsement of any particular manufacturer or product. The design engineer should evaluate each product for its suitability to the particular project being designed, and is encouraged to consult periodically with NJDEP to determine whether additional products or technologies have been certified since the creation of this document.

B. Arrange the Manufactured Water Quality devices in accordance with the

New Jersey Stormwater Management BMP Manual’s “Guidelines for

Arranging BMPs in a Series”. The design of the water quality device needs to

ensure that it is located such that the structure can be easily maintained

(i.e. the device is not located in the middle of a busy roadway.)

C. Selection of the appropriate water quality device should take the frequency

of the maintenance into consideration. Maintenance of the device, once it is

determined to be performing as designed, should be performed at most

twice a year and at least once a year. The use of replacement filters is to be

discouraged.

http://www/

Drainage Design

D. A maintenance plan shall be developed for the manufactured water quality

device. The maintenance plan shall at a minimum contain specific

preventative maintenance task and schedules and be in compliance with

N.C.A.C. 7:8-5.8 and the Maintenance Guidelines for stormwater

management measures in the New Jersey Stormwater Best Management

Practices Manual.

12.4 Scour Considerations

Scour is to be evaluated for stream encroachment and outlet pipe protection of

culverts and storm sewer pipes. For stream encroachment, substructure

foundations need to be investigated for scour in accordance with the AASHTO

LRFD NJDOT Design Manual for Bridges and Structures, Division 1, Section 46.

The investigation consists of determining what the substructures are founded

on; how deep the foundation is; and a decision on whether potential scour will

endanger the substructure’s integrity. Local scour and contraction scour need to

be considered.

Scour is to be evaluated utilizing site-specific geotechnical information (e.g.,

soil types, d50, etc.). The following data should be assessed in determining

geotechnical impacts on the scour analysis:

1.) Review subsurface information that is provided in the Geotechnical

Report.

2.) Evaluate historic scour related conditions and potential scour holes at

the bridge site.

3.) Soil classification – Based on laboratory tests for grain size samples,

classify the soil.

Scour depths and appropriate countermeasures can be determined through the

use of the Hydraulic Engineering Circular No. 18 (HEC-18), “Evaluating Scour at

Bridges”, HEC-20, “Stream Stability at Highway Structures”, and HEC-23,

“Bridge Scour and Stream Instability Countermeasures”.

Outlet protection for culverts and storm sewer pipes should be designed in

accordance with Section 9.0, Conduit Outlet Protection.

Drainage Design

13.0 Sample Hydrologic and Hydraulic Calculations

A sample storm sewer hydraulic computations and hydrologic pond design

demonstrate the design procedure for a simple storm sewer system and pond

as shown on Figure 13-1. For this sample, design a new land service highway

through a meadow in Woodbine, NJ.

Obtain Tc for overland flow to inlets 1, 3 and 4 (based on the hydraulically most

distant point) (See Manual Section 3.2) Obtain Tc from Figure 13-2.

Inlet #1

Ground Cover is grass

Overland flow length = 800 ft

Elevation at farthest point = 112 ft

Elevation at inlet = 98 ft

H = 14 ft

From Figure 13-2, (overland flow Tc)

Tc = 6 minutes, multiply by 2 for grass

Tc = 12 minutes

Inlet #3

Ground cover is grass

Overland flow length = 980 ft


Elevation at inlet = 96 ft

H= 2 ft

From Figure 13-2

Tc = 17 minutes, multiply by 2 for grass

Tc = 34 minutes

Inlet #4

Ground Cover is grass

Overland flow length (farthest point from channel) = 480 ft


Elevation of channel invert = 102 ft

H= 16 ft

From Figure 13-2

Tc = 3.2 minutes

Multiply by 2 for grass

Tc =6.4 mins.

Tt through channel:

L=330 ft

H=102 ft – 93 ft= 9 ft

From Figure 13-2

Tc =2.5 min.

Total Tc = 6.4 mins.+ 2.5 mins. = 8.9 minutes, use 10 minute minimum Tc

Drainage Design

Figure -13-1

Drainage Design

Figure 13-2

Drainage Design

13.1 Sample Hydraulic Calculations

Using Rational formula, find 10-year runoff to each inlet: (See Manual

Section 3.3)

Q=CIA

Refer to Table 3-2 for runoff coefficients (“C”), using soil group B

Using Figure 3-1, locate the project in Woodbine, NJ. The project is located in

the East Region, therefore use Figure 3-4 to obtain the rain fall intensity.

Obtain rainfall intensity (I) from Figure 13-3

Inlet Tc (min) I (in/hr)

1 12 5.0

2 10 5.3

3 34 3.0

4 10 5.3

Inlet #1

Q1 = (0.25)(5.0 in/hr)(2.471 acres) = 3.09 cfs

Inlet #2

Q2 = (0.99)(5.3 in/hr)(0.148 acre) = 0.78 cfs

Inlet #3

Q3 = (0.148x0.99 + 0.494x0.25)(3.0 in/hr)(0.642 acre) = 0.81 cfs

0.642

Inlet #4

Q4= (0.148x0.99 + 1.0x0.25)(5.3 in/hr)(1.148 acre)= 2.10 cfs

1.148

Compute gutter spread width, intercepted flow, bypass flow and

efficiency for each roadway inlet: (See Manual Sections 5.5 and 5.7)

Inlet #2 (type D-1 inlet)

Q= 0.78 cfs

Sx=0.04

S= 0.03

n=0.013

Tall=4 ft(inside shldr. width) + 4 ft(1/3 of inside lane)=8 ft (allowable spread)

Drainage Design

Figure 13-3

Drainage Design

Using a modification of the Manning equation, obtain gutter spread width:

Q = 0.56 Sx1.67S0.5T2.67, solve for T (Section 5.4)

n

T2.67= 0.78

(0.56/0.013)(0.04)5/3(0.03)1/2

T=3.20 ft < Tall of 8 ft, OK

y=TSx (Section 5.4)

y=3.20 ft (0.04) = 0.128 ft

For the standard NJDOT bicycle safe grate, the following equation shall be used

to obtain inlet interception:

Qi = 16.88(y)1.54(S)0.233 (Section 5.7)

Sx0.276

Qi = 16.88(0.128)1.54(0.03)0.233 = 0.76 cfs

0.040.276

Determine bypass runoff = total runoff -intercepted runoff

Bypass flow = 0.78 – 0.76 = 0.02 cfs

(0.02 cfs would bypass to downstream inlet)

Check inlet efficiency:

0.76 cfs = 0.97 > 75% , OK

0.78 cfs

Inlet #3 (type B inlet)

Q=0.81 cfs

sx=0.04

S=0.03

Tall=10 ft

Using above equation to solve for T:

T2.67= 0.81

(0.56/0.013)(0.04)5/3(0.03)1/2


Compute inlet interception:

When T=3.24 ft, y=3.24(0.04) = 0.130 ft

Qi = 16.88(0.130)1.54(0.03)0.233 = 0.78 cfs

0.040.276

Bypass flow = 0.81 - 0.78 = 0.03 cfs

(0.03 cfs will bypass to inlet #4)


0.78 = 0.96 > 0.75, OK

0.81

Inlet #4 (type B inlet)

Drainage Design

Q=2.10 cfs + 0.03 cfs (bypass from inlet #3) = 2.13 cfs

sx=0.04

S=0.025

Tall=10 ft

Using above equation to solve for T:

T2.67= 2.13

(0.56 /0.013)(0.04)5/3 (0.025) 1/2



When T=4.83 ft, y=4.83(0.04) = 0.193 ft

Qi = 16.88(0.193) 1.54 (0.025) 0.233 = 1.38 cfs

0.04 0.276


1.38 = 0.65<0.75

2.13

Since the efficiency is <75%, this inlet should be moved upstream.

When the spread width exceeds the shoulder width, the excess runoff extends

into the adjacent lane, which typically has a different cross slope than the

shoulder. The following example presented the computational procedure to

determine the spread.

Obtain spread width for a composite gutter section:

Say conditions for inlet #2 are such that:

Q= 1.836 cfs

Sx=0.04ft/ft

S= 0.005 ft/ft

n=0.013

T (allowable) =5.0 ft(inside shldr. width) + 4.0 ft(1/3 of inside lane)= 9.0 ft

Using above equation:

T2.67= 1.836

(0.56/0.013)(0.04)5/3(0.005)1/2

T= 6.17 ft

Inside shoulder width is 5 ft, therefore, spread is beyond shoulder into adjacent

through lane. Since the cross slope of the through lane differs from that of the

shoulder, a composite gutter spread calculation must be performed to

determine correct spread width.

Drainage Design

Initially, a depth is assumed (y1). Qx,Qy and Qz are then calculated using the

above equation. The flow contained in the composite section (Qt) is equal to Qx

+ Qz - Qy. This process is repeated until Qt = Q (actual flow in the gutter). T

(actual spread width) is equal to Tx+ Tz - Ty.

Drainage Design

Given T1= 5 ft, y3=5 ft(0.04)=0.20 ft

Find Qx (Triangle 1,2,4)

Assume y1 = 0.25 ft , Tx = 6.25 ft

Qx = 0.56 (0.04)5/3(0.005)1/2(6.25 ft)2.67

0.013

Qx=1.90 cfs

Find Qz (Triangle 3,5,6)

Tz= (y1-y3) = 0.05 = 3.33 ft

0.015 0.015

Qz = 0.56 (0.015)5/3(0.005)1/2(3.33)2.67

0.013

Qz = 0.07cfs

Find Qy (Triangle 3,4,6)

Ty= (y1-y3) = 1.25 ft

0.04

Qy = 0.56 (0.04)5/3(0.005)1/2(1.25)2.67

0.013

Qy=0.03cfs

Qt= 1.90 cfs+0.07 cfs - 0.03 cfs= 1.94 cfs

Qt= Q, therefore, assumed depth is correct

Calculate T (actual spread width) (T1 + Tz - Ty)

T=6.25 ft+3.33 ft – 1.25 ft= 8.33 ft

T=8.33 ft<9 ft, OK


Qi = 16.88(0.25)1.54(0.005)0.233 = 1.41 cfs

0.040.276


1.41 = 0.77 0.75, OK

1.836

Obtain gutter spread width for inlet at low point: (See Manual Section

5.8)

Utilize same conditions at inlet #4, except s=0% (sag condition)

Q = 20.88(y)1.5 (for weir flow)

Solving for y:

y = Q0.67 = 2.100.67

7.58 7.58

y=0.22 ft (Less than 0.75 ft, therefore use of weir equation is acceptable)

T = d (d=y)

Sx

When d= 0.22 ft, T= 0.22 = 5.50 ft

0.04

Drainage Design

T=5.50 < Tall of 10 ft , OK

Compute storm drain pipe sizes for network using sample forms at end

of this subsection. (See Manual Sections 6.4 and 6.5)

Backup Computations for Pipe Travel Time for Figure 6-2

Find Tc for pipe flow for partly full pipe (pipe 1-3):

(See Manual Section 3.5)

From column 12 - Q=3.11 cfs

From column 15 - Qc = 4.95 cfs

3.11 = 0.63 (63% full)

4.95

From Concrete Pipe Design Manual chart, “Relative Velocity and Flow in Circular

Pipe”, at 63% full, v=1.06 of full velocity.

vfull = 4.03 ft/s , vdes = 4.03 ft/s(1.06)=4.27 ft/s

Tt = 197 ft = 0.77 min., Tc = 12.77 min. (12 min. to Junction 1 + 0.77

4.27 ft/s min. travel time in pipe)

Since Tc at inlet 3 from overland flow is 34.0 min. > 12.77 min., use 34.0 min.

Drainage Design

PRELIMINARY STORM

DRAIN COMPUTATION FORM

[SAMPLE]

FIGURE 13-4


Section:


Station and Offset

(1)

L (ft)

Drainage Area

“A”

(Acres)

Runoff

Coef-ficient

“C”

“A” x “C” Flow Time

“Tc”

(min.)

Rainfall

"I"

in/hr

Total

Chapter 3 R

unoff

Q=CIA

ft3/S

Dia.

Pipe

ft

Slope

ft/ft

Capacity

Full

ft3/S

Velocity

ft/s

Invert Elevation

Junction

From

Junction

To

Incre-

ment

Total

Incre-

ment

Total

Overland

To

Inlet

In

U/S

Pipe

Cum.

Total in

Pipe*

Flowing

Full

Design

Flow

U/S

End

D/S

End

(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

1 3 197 2.471 2.471 0.25 0.618 0.618 12.0 -- 12.0 5.0 3.09 15 0.005 4.95 4.03 4.38 88.68 87.70

2 3 49 0.148 0.148 0.99 0.147 0.147 10.0 -- 10.0 5.3 0.779 15 0.005 4.95 4.03 3.03 91.37 91.14

3 4 197 0.494 0.25 0.124 34.0

0.148 0.99 0.147 0.75 34.0 3.0

3.261 1.036 (Line 1-3)

3.11 15 0.02 9.90 8.06 7.39 87.60 83.66

4 5 164 1.0 0.25 0.25 10.0

0.148 0.99 0.147 0.39

4.41 1.433 34.39 3.0 4.30 15 0.006 5.42 4.42 4.97 83.56 82.58

5 6 49 -- 4.41 1.433 0.37 34.76 3.0 4.30 18 0.005 8.05 4.55 4.73 82.35 82.09

For Time of Concentration, use larger of overland flow to inlet or cumulative time in pipe.

Drainage Design

HYDRAULIC GRADE LINE

COMPUTATION FORM

[SAMPLE]

FIGURE 13-5


Section:


Station & Offset

(1)

(2) Q

(3) V

(4) R

(5) L

(6) n

(7) h

(8) Hf (9)

He (10)

Hi (11)

Hs (12)

Ht (13)

TW (14)

HGL (15)

TOS (16)

CL (17)

Junction From

Junction To

Dia. ft

Flow ft3/S

Vel. ft/s

Hydraulic radius

ft

Length ft

Manning's

Vel. Head

ft

Fric. Loss

ft

Exit Loss

ft

Entr. Loss

ft

Struct. Loss*

ft

Total Head

Loss

ft

Tail- water

Elev.

ft

Head- water

Elev.

ft

Top of Struct.

Elev.

ft

TOS- HGL

ft

6 (outlet) 5 18 4.30 2.43 0.375 49 0.012 0.092 0.069 0.092 -- 0.01 0.171 84.08 84.251 92.52 8.269

5 4 15 4.30 3.50 0.312 164 0.012 0.190 0.616 -- -- 0.08 0.696 84.251 84.947 92.85 7.903

4 3 15 3.11 2.53 0.312 197 0.012 0.099 0.387 -- -- 0.03 0.417 84.947 85.364 96.46 11.096

3 2 15 0.779 0.63 0.312 49 0.012 0.006 0.006 -- 0 -- 0.006 85.364 85.37 96.79 11.42

3 1 15 3.09 2.52 0.312 197 0.012 0.099 0.384 -- 0.020 -- 0.404 85.37 85.774 97.44 11.666

h = Velocity head, = (V)2/2g Hi = Entrance Loss = Ki(V)2/2g Refer to Table 6-3 for values of Ki

Hf = Friction Loss, = 29.1N2L X (V)2 He = Exit Loss, He = (V)2/2g

R1.33 2g * For structural (junction) losses in inlets, manholes, see Figure 13-6.

Drainage Design

STRUCTURAL AND BEND

LOSS COMPUTATION FORM

[SAMPLE]

FIGURE 13-6


Section:


(1)

(2) Q

(3) v

(4) v2 2g

(5)

(6)

(7)

Ks (8)

Hs (9)

A (10)

Kb (11)

Hb (12)

Hs + Hb (13)

Junction

Station &

Offset

Downstream

Dia.

ft

Downstream

Flow

ft3/S

Downstream

Velocity

ft/s

Velocity

Head

ft

Junction

Type

(L,N or O)

Flow

Type

(P or O)

Structural

Loss

Coeff.

Structural

Loss

ft

Angle

deg.

Bend

Factor

Bend Loss

ft

Structural

Loss +

Bend Loss

ft

6 -- -- 0 0 -- -- -- -- -- -- --

5 18 4.30 2.43 0.09 N P 0.3 0.03 11 0.15 0.01 0.04

4 15 4.30 3.50 0.19 N P 0.3 0.06 37 0.41 0.08 0.14

3 15 3.11 2.53 0.10 L P 1.0 0.10 28 0.33 0.03 0.13

3.11 -- -- -- -- -- -- --

2 15 0.779 0.44 0.003 N P -- -- -- -- -- --

1 15 3.09 1.75 0.05 N P -- -- -- -- -- --

Hs = Structural Loss = KS X (V)2 , Ks from Table 6-4 NOTES: 1) Junction Type 2) Flow Type

2g L = with Lateral P = Pressure N = with No Lateral O = Open Channel Hb = Bend Loss = Kb X (V)2 , Kb from Figure 7-1 O = with Opposed Laterals

2g

Drainage Design

13.2 Sample Hydrologic Calculations

For the same project, design a pond so that the post-construction peak runoff

rates for the 2-year, 10-year, and 100-year storm events are 50%, 75%, and

80%, respectively, of the pre-construction rates. Due to the complexity of

designing a pond, use of computer software is encouraged. In this example,

software was used and the input and output is summarized below.

Determine what the pre-construction and post-construction discharges are

without a detention pond.

Using TR-55 the discharges are the following:

Area

Name

Area

(Acre)

Curve

Number

2-Year Flow

(cfs)

10-Year Flow

(cfs)

100-Year Flow

(cfs)

Existing 1 2.471 58 0.51 2.72 8.96

Existing 2 0.148 58 0.03 0.16 0.54

Existing 3 0.642 58 0.13 .71 2.33

Existing 4 1.148 58 0.24 1.27 4.16

Total 4.409 -- 0.92 4.86 15.99

Proposed 1 2.471 58 0.51 2.72 8.96

Proposed 2 0.148 98 0.40 0.63 1.09

Proposed 3 0.642 67 0.26 .76 1.97

Proposed 4 1.148 63 0.47 1.73 4.90

Total 4.409 -- 1.45 5.52 16.20

With the aid of computer software, design a pond so that the post-construction

peak runoff rates for the 2-year, 10-year, and 100-year storm events are 50%,

75%, and 80%, respectively, of the pre-construction rates. The pond should

have design flows as follows:

Inflow

(cfs)

Design Discharge

(cfs)

2-year 0.92 0.46

10-year 4.86 3.65

100-Year 15.99 12.79

Design a pond with a bottom length of 75 feet, bottom width of 40 feet and

with 2:1 side slopes. The pond will be located as shown on Figure 13-1. The

outlet structure will be a riser inlet box with a 3” orifice at elevation 82.0 feet

and an overflow weir at elevation 88.0 feet. The outlet pipe from the outlet

structure is an 18” reinforced concrete pipe. Note that different pond and outlet

structure configuration may need to be tried for the pond to perform at the

design discharge. Use of a pond sizing wizard may be helpful in determining a

starting point. If the required pond size is too large for the proposed project,

multiple smaller ponds may be used for detention. The pond, as designed, has

the following discharges:

Discharge

(cfs)

% of Pre-

Construction Rates Check

2-year 0.16 17 OK

10-Year 3.60 74 OK

100-year 11.02 69 OK

Drainage Design

References

1. American Iron and Steel Institute, HANDBOOK OF STEEL DRAINAGE AND HIGHWAY

CONSTRUCTION PRODUCTS, FIFTH EDITION, 150 East 42nd Street, New York, 1994.

Harrison, L.J., Morris, J.L., Normann, J.M., and Johnson, F.L.

2. American Concrete Pipe Association, CONCRETE PIPE DESIGN MANUAL, Revised April

2004.

3. "Aluminum Storm Sewers - Corlix/Riveted Pipe/Structural Plate", 4th Edition.

4. Federal Highway Administration, DESIGN CHARTS FOR OPEN-CHANNEL FLOW, U.S.

Government Printing Office, Washington, DC. 1961. (Hydraulic Design Series No. 3).

5. "Drainage of Highway Pavement", FHWA, U.S. Government Printing Office,

Washington, DC. (HEC No. 12),March 1984.

6. NOAA Atlas 14, Volume 2, Precipitation-Frequency of the United States, June 2,

2005.

7. "Design of Urban Highway Drainage-The State of the Art", USDOT, FHWA - TS-79-

225, August 1979.

8. FHWA, "Hydraulic Design of Highway Culverts", U.S. Government Printing Office,

Washington, DC. September 2004. (HDS No. 5).

9. New Jersey Department of Environmental Protection, "Technical Manual for Land Use

Regulation Program, Bureau of Inland and Coastal Regulations, Stream

Encroachment Permits", Revised September 1995.

10. "FUNDAMENTAL OF URBAN RUNOFF MANAGEMENT: Technical and Institutional

Issues", Terrene Institute, Washington, D.C., Horner, R. Skupien, J.J., Livingston,

E.H., and Shaver, H.E., August 1994.

11. New Jersey Department of Environmental Protection, "New Jersey Stormwater Best

Management Practices Manual", April 2004.

12. “Stormwater Management Rule”, N.J.A.C. 7.8

13. “Antidegradation Polices”, N.J.A.C. 7:9B-4

14. New Jersey Department of Environmental Protection, “Highway Agency Stormwater,

Guidance Document”, August 2004

15. New Jersey Department of Transportation, “Bridge and Structures Design Manual”,

Section 46, Scour at Bridges.

16. Federal Highway Administration, “Evaluating Sour at Bridges, 4th Edition”, May

2001 (HEC-18).

17. Federal Highway Administration, “Stream Stability at Highway Structures, Third

Edition”, March 2001, (HEC-20).

18. Federal Highway Administration, “Bridge Scour and Stream Instability

Countermeasures, 2nd Edition”, March 2001, (HEC-23).

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